Semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device that can be miniaturized or highly integrated is provided. The semiconductor device includes a first insulator over a substrate; an oxide over the first insulator; a second insulator over the oxide; a first conductor over the second insulator; a third insulator over the first conductor; a fourth insulator in contact with a side surface of the second insulator, a side surface of the first conductor, and a side surface of the third insulator; a fifth insulator in contact with a top surface of the oxide and a side surface of the fourth insulator; and a second conductor in contact with the top surface of the oxide and the fifth insulator. The level of the top surface of the fourth insulator is higher than the level of the top surface of the fifth insulator.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductor device and a manufacturing method thereof. Another embodiment of the present invention relates to a semiconductor wafer, a module, and an electronic device.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each an embodiment of a semiconductor device. A display device (e.g., a liquid crystal display device and a light-emitting display device), a projection device, a lighting device, an electro-optical device, a power storage device, a memory device, a semiconductor circuit, an imaging device, an electronic device, and the like may include a semiconductor device.

Note that one embodiment of the present invention is not limited to the above technical field. One embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. Furthermore, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter.

2. Description of the Related Art

A technique of fabricating a transistor using a semiconductor thin film has attracted attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film that can be used in a transistor. As another material, an oxide semiconductor has been attracting attention.

Techniques have been disclosed by each of which a display device is manufactured using a transistor whose active layer is formed of zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor (see Patent Documents 1 and 2), for example.

In recent years, a technique has been disclosed by which an integrated circuit of a memory device is manufactured using a transistor including an oxide semiconductor (see Patent Document 3). Furthermore, not only memory devices but also arithmetic devices and the like are manufactured using transistors including oxide semiconductors.

REFERENCE Patent Document [Patent Document 1] Japanese Published Patent Application No. 2007-123861 [Patent Document 2] Japanese Published Patent Application No. 2007-096055 [Patent Document 3] Japanese Published Patent Application No. 2011-119674 SUMMARY OF THE INVENTION

The integration of integrated circuits and miniaturization of transistors have progressed in accordance with an increase in performance and a reduction in size and weight of electronic devices. Thus, the process rule for fabricating a transistor has decreased year by year to 45 nm, 32 nm, and then 22 nm. Accordingly, transistors with fine structures including oxide semiconductors are required to have good electrical characteristics as designed.

An object of one embodiment of the present invention is to provide a semiconductor device that can be miniaturized or highly integrated. Another object of one embodiment of the present invention is to provide a semiconductor device having good electrical characteristics. Another object of one embodiment of the present invention is to provide a semiconductor device with low off-state current. Another object of one embodiment of the present invention is to provide a transistor with high on-state current. Another object of one embodiment of the present invention is to provide a highly reliable semiconductor device. Another object of one embodiment of the present invention is to provide a semiconductor device with low power consumption. Another object of one embodiment of the present invention is to provide a semiconductor device that can be manufactured with high productivity.

Another object of one embodiment of the present invention is to provide a semiconductor device capable of retaining data for a long time. Another object of one embodiment of the present invention is to provide a semiconductor device capable of high-speed data writing. Another object of one embodiment of the present invention is to provide a semiconductor device with high design flexibility. Another object of one embodiment of the present invention is to provide a low-power semiconductor device. Another object of one embodiment of the present invention is to provide a novel semiconductor device.

Note that the description of these objects does not preclude the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects are apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

One embodiment of the present invention is a transistor including an oxide semiconductor. Insulators are positioned in contact with the top surface and a side surface of a gate electrode and a side surface of a gate insulating film. The insulators are preferably deposited by an atomic layer deposition (ALD) method. When the ALD method is employed for the deposition, an insulator formed of a film with good coverage or a dense film can be obtained. The provision of such an insulator in contact with the side surface of the gate insulating film can prevent outward diffusion of oxygen contained in the gate insulating film and entry of impurities such as water or hydrogen into the gate insulating film. Furthermore, since the insulators are positioned to cover the top surface and the side surface of the gate electrode, oxidation of the gate electrode can be prevented.

According to one embodiment of the present invention, an insulator is provided, an insulating film is provided over the insulator, contact holes are formed in the insulating film to expose the insulator, and a source electrode and a drain electrode that are connected to a source region and a drain region, respectively, of the transistor are formed in the contact holes, whereby the source electrode and the drain electrode become close to each other; thus, a transistor can be miniaturized. Furthermore, the parasitic resistance of the source region and the drain region can be reduced, so that excellent electrical characteristics can be obtained.

One embodiment of the present invention is a semiconductor device including a first insulator over a substrate; an oxide over the first insulator; a second insulator over the oxide; a first conductor over the second insulator; a third insulator over the first conductor; a fourth insulator in contact with a side surface of the second insulator, a side surface of the first conductor, and a side surface of the third insulator; a fifth insulator in contact with the top surface of the oxide and a side surface of the fourth insulator; and a second conductor in contact with the top surface of the oxide and the fifth insulator. The level of the top surface of the fourth insulator is higher than the level of the top surface of the fifth insulator.

Another embodiment of the present invention is the semiconductor device in which the angle formed between the side surface of the first conductor and the bottom surface of the oxide is greater than or equal to 80° and less than or equal to 100°.

Another embodiment of the present invention is the semiconductor device in which upper corners of the oxide are rounded in a cross section, and the curvature radius of each of the rounded corners is greater than or equal to one-fourth of the thickness of the oxide and less than or equal to the thickness of the oxide.

Another embodiment of the present invention is the semiconductor device in which the second conductor may be in contact with the corner, the top surface of the oxide, and the fifth insulator.

Another embodiment of the present invention is the semiconductor device in which first electrostatic capacitance is formed between the oxide and the first conductor, second electrostatic capacitance is formed between the first conductor and the second conductor, and the second electrostatic capacitance is smaller than the first electrostatic capacitance.

Another embodiment of the present invention is the semiconductor device in which the oxide includes In, an element M (Al, Ga, Y, or Sn), and Zn.

Another embodiment of the present invention is the semiconductor device in which the oxide includes a first region and a second region overlapping with the second insulator, at least part of the first region is in contact with the fifth insulator and the second conductor, and a concentration of at least one of hydrogen and nitrogen is preferably higher in the first region than in the second region.

Another embodiment of the present invention is the semiconductor device in which the second region includes a region overlapping with the fourth insulator and the second insulator.

Another embodiment of the present invention is the semiconductor device in which the fourth insulator includes aluminum oxide or hafnium oxide.

Another embodiment of the present invention is the semiconductor device in which the third insulator includes aluminum oxide or hafnium oxide.

Another embodiment of the present invention is the semiconductor device in which the first conductor includes a conductive oxide.

Another embodiment of the present invention is the semiconductor device in which the fifth insulator includes hydrogen and/or nitrogen.

Another embodiment of the present invention is a method for manufacturing a semiconductor device including forming a first insulator over a substrate; forming an oxide layer over the first insulator; forming a first insulating film, a first conductive film, and a second insulating film in this order over the oxide layer; etching the first insulating film, the first conductive film, and the second insulating film to form a second insulator, a first conductor, and a third insulator; forming a third insulating film by an ALD method to cover the first insulator, the oxide layer, the second insulator, the first conductor, and the third insulator; performing first dry etching treatment on the third insulating film to form a fourth insulator in contact with a side surface of the second insulator, a side surface of the first conductor, and a side surface of the third insulator; forming a fourth insulating film by a PECVD method to cover the first insulator, the oxide layer, the fourth insulator, and the third insulator; performing second dry etching treatment on the fourth insulating film to form a fifth insulator in contact with a side surface of the fourth insulator; forming a sixth insulator over the first insulator, the oxide layer, the third insulator, and the fifth insulator; forming a first opening and a second opening in the sixth insulator so that part of the fifth insulator and at least part of the top surface and a side surface of the oxide layer are exposed in each of the first opening and the second opening; forming a second conductor to fill the first opening; and forming a third conductor to fill the second opening.

According to one embodiment of the present invention, a semiconductor device that can be miniaturized or highly integrated, a semiconductor device having good electrical characteristics, a semiconductor device with low off-state current, a transistor with high on-state current, a highly reliable semiconductor device, a semiconductor device with low power consumption, or a semiconductor device that can be manufactured with high productivity can be provided.

According to one embodiment of the present invention, a semiconductor device capable of retaining data for a long time, a semiconductor device capable of high-speed data writing, a semiconductor device with high design flexibility, a low-power semiconductor device, or a novel semiconductor device can be provided.

Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects are apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views of a semiconductor device of one embodiment of the present invention;

FIGS. 2A to 2C are a top view and cross-sectional views of a semiconductor device of one embodiment of the present invention;

FIGS. 3A to 3C are a top view and cross-sectional views of a semiconductor device of one embodiment of the present invention;

FIGS. 4A and 4B are each a cross-sectional view of a semiconductor device of one embodiment of the present invention;

FIGS. 5A and 5B are each a cross-sectional view of a semiconductor device of one embodiment of the present invention;

FIG. 6 is a cross-sectional view of a semiconductor device of one embodiment of the present invention;

FIGS. 7A and 7B are each a cross-sectional view of a semiconductor device of one embodiment of the present invention;

FIGS. 8A to 8C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 9A to 9C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 10A to 10C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 11A to 11C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 12A to 12C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 13A to 13C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 14A to 14C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 15A to 15C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 16A to 16C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 17A to 17C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 18A to 18C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 19A to 19C are a top view and cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIG. 20 is a cross-sectional view illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 21A to 21C each show an atomic ratio range of a metal oxide of the present invention;

FIG. 22 is a cross-sectional view illustrating a structure of a memory device of one embodiment of the present invention;

FIG. 23 is a cross-sectional view illustrating a structure of a memory device of one embodiment of the present invention;

FIG. 24 is a cross-sectional view illustrating a structure of a memory device of one embodiment of the present invention;

FIG. 25 is a cross-sectional view illustrating a structure of a memory device of one embodiment of the present invention;

FIG. 26 is a block diagram showing a configuration example of a memory device of one embodiment of the present invention;

FIGS. 27A and 27B are a block diagram and a circuit diagram showing a configuration example of a memory device of one embodiment of the present invention;

FIGS. 28A to 28C are block diagrams showing a configuration example of a semiconductor device of one embodiment of the present invention;

FIGS. 29A and 29B are a block diagram and a circuit diagram showing a configuration example of a semiconductor device of one embodiment of the present invention, and FIG. 29C is a timing chart showing an operation example of the semiconductor device;

FIG. 30 is a block diagram showing a configuration example of a semiconductor device of one embodiment of the present invention;

FIG. 31A is a circuit diagram showing a configuration example of a semiconductor device of one embodiment of the present invention, and FIG. 31B is a timing chart showing an operation example of a semiconductor device;

FIG. 32 is a block diagram illustrating a semiconductor device of one embodiment of the present invention;

FIG. 33 is a circuit diagram of a semiconductor device of one embodiment of the present invention;

FIGS. 34A and 34B are top views of a semiconductor wafer of one embodiment of the present invention;

FIG. 35A is a flowchart showing a manufacturing process example of an electronic component, and FIG. 35B is a schematic perspective view of the electronic component; and

FIGS. 36A to 36F each illustrate an electronic device of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments are described with reference to drawings. Note that the embodiments can be implemented in various modes, and it is readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments.

In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Thus, the size, the layer thickness, or the region is not limited to the illustrated scale. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to shapes or values shown in the drawings. For example, in the actual manufacturing process, a layer, a resist mask, or the like might be unintentionally reduced in size by treatment such as etching, which is not illustrated in some cases for easy understanding. In the drawings, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and explanation thereof is not repeated in some cases. Furthermore, the same hatching pattern is applied to portions having similar functions, and the portions are not denoted by reference numerals in some cases.

Especially in a top view (also referred to as a “plan view”), a perspective view, or the like, some components might not be illustrated for easy understanding of the invention. In addition, some hidden lines and the like might not be shown.

Note that the ordinal numbers such as “first”, “second”, and the like in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. Thus, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those that specify one embodiment of the present invention.

In this specification, terms for describing placement, such as “over”, “above”, “under”, and “below”, are used for convenience in describing a positional relationship between components with reference to drawings. Furthermore, the positional relationship between components is changed as appropriate in accordance with a direction in which each component is described. Thus, description of the positional relationship can be rephrased appropriately according to the situation, without being limited by the terms used in this specification.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor includes a channel formation region between the drain (a drain terminal, a drain region, or a drain electrode) and the source (a source terminal, a source region, or a source electrode) and current can flow between the source and the drain through the channel formation region. Note that in this specification and the like, a channel formation region refers to a region through which current mainly flows.

Furthermore, functions of the source and the drain might be switched when the transistor has opposite polarity or a direction of current flow is changed in circuit operation, for example. Thus, the terms “source” and “drain” can be interchanged with each other in this specification and the like in some cases.

Note that the channel length refers to, for example, the distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a plan view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not fixed to one value in some cases. Thus, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

The channel width refers to, for example, the length of a channel formation region perpendicular to the channel length direction in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other, or a region where a channel is formed in a top view of the transistor. In one transistor, channel widths in all regions are not necessarily the same. In other words, the channel width of one transistor is not fixed to one value in some cases. Thus, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

Note that depending on transistor structures, a channel width in a region where a channel is actually formed (hereinafter referred to as an “effective channel width”) is different from a channel width shown in a top view of a transistor (hereinafter referred to as an “apparent channel width”) in some cases. For example, in a transistor having a gate electrode covering a side surface of a semiconductor, an effective channel width is greater than an apparent channel width, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a gate electrode covering a side surface of a semiconductor, the proportion of a channel formation region formed in a side surface of a semiconductor is increased. In that case, an effective channel width is greater than an apparent channel width.

In such a case, an effective channel width is difficult to measure in some cases. For example, to estimate an effective channel width from a design value, it is necessary to assume that the shape of a semiconductor is known as an assumption condition. Accordingly, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure an effective channel width accurately.

Thus, in this specification, an apparent channel width is referred to as a surrounded channel width (SCW) in some cases. Furthermore, in this specification, in the case where the term “channel width” is simply used, it may represent a surrounded channel width or an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may represent an effective channel width. Note that a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by analyzing a cross-sectional TEM image and the like.

Note that an impurity in a semiconductor refers to, for example, elements other than the main components of a semiconductor. For example, an element with a concentration lower than 0.1 atomic % can be regarded as an impurity. When an impurity is contained, the density of states (DOS) in a semiconductor may be increased, or the crystallinity may be decreased. In the case where the semiconductor is an oxide semiconductor, examples of an impurity that changes characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components of the oxide semiconductor; there are hydrogen, lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. For an oxide semiconductor, water also serves as an impurity in some cases. For an oxide semiconductor, entry of impurities may lead to formation of oxygen vacancies, for example. Furthermore, when the semiconductor is silicon, examples of an impurity that changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.

In this specification and the like, a silicon oxynitride film contains more oxygen than nitrogen. A silicon oxynitride film preferably contains, for example, oxygen, nitrogen, silicon, and hydrogen in the ranges of 55 atomic % to 65 atomic % inclusive, 1 atomic % to 20 atomic % inclusive, 25 atomic % to 35 atomic % inclusive, and 0.1 atomic % to 10 atomic % inclusive, respectively. A silicon nitride oxide film contains more nitrogen than oxygen. A silicon nitride oxide film preferably contains nitrogen, oxygen, silicon, and hydrogen in the ranges of 55 atomic % to 65 atomic % inclusive, 1 atomic % to 20 atomic % inclusive, 25 atomic % to 35 atomic % inclusive, and 0.1 atomic % to 10 atomic % inclusive, respectively.

In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

In addition, in this specification and the like, the term “insulator” can be replaced with the term “insulating film” or “insulating layer”. Moreover, the term “conductor” can be replaced with the term “conductive film” or “conductive layer”. Furthermore, the term “semiconductor” can be replaced with the term “semiconductor film” or “semiconductor layer”.

Furthermore, unless otherwise specified, transistors described in this specification and the like are field effect transistors. Unless otherwise specified, transistors described in this specification and the like are n-channel transistors. Thus, unless otherwise specified, the threshold voltage (also referred to as “Vth”) is larger than 0 V.

In this specification and the like, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, the term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. The term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. In addition, the term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system.

Note that in this specification, a barrier film refers to a film having a function of inhibiting the penetration of oxygen and impurities such as hydrogen. The barrier film that has conductivity may be referred to as a conductive barrier film.

In this specification and the like, a metal oxide means an oxide of metal in a broad sense. Metal oxides are classified into an oxide insulator, an oxide conductor (including a transparent oxide conductor), an oxide semiconductor (also simply referred to as an OS), and the like. For example, a metal oxide used in an active layer of a transistor is called an oxide semiconductor in some cases. In other words, an OS field effect transistor (FET) is a transistor including an oxide or an oxide semiconductor.

Embodiment 1 <Structure Example of Semiconductor Device>

An example of a semiconductor device including a transistor 1000 of one embodiment of the present invention is described below.

FIG. 1A is a top view of a semiconductor device including the transistor 1000. FIG. 1B is a cross-sectional view taken along the dashed-dotted line A1-A2 in FIG. 1A, which corresponds to a cross-sectional view in the channel length direction of the transistor 1000. FIG. 1C is a cross-sectional view taken along the dashed-dotted line A3-A4 in FIG. 1A, which corresponds to a cross-sectional view in the channel width direction of the transistor 1000. Note that for simplification of the drawing, some components in the top view in FIG. 1A are not illustrated.

As illustrated in FIGS. 1A to 1C, the transistor 1000 includes an insulator 402 provided over a substrate (not shown); an oxide 406 a provided over the insulator 402; an oxide 406 b provided in contact with at least a portion of the top surface of the oxide 406 a; an insulator 410 in contact with part of a side surface of the oxide 406 a and part of a side surface of the oxide 406 b; an insulator 420 in contact with part of a side surface of the oxide 406 b and a side surface of the insulator 410; an insulator 412 provided over the oxide 406 b; a conductor 404 a provided over the insulator 412; a conductor 404 b provided over the conductor 404 a; an insulator 419 provided over the conductor 404 b, an insulator 418 provided in contact with side surfaces of the insulator 412, the conductor 404 a, the conductor 404 b, and the insulator 419; and an insulator 409 provided in contact with a side surface of the insulator 418. As illustrated in FIG. 1B, it is preferable that the top surface of the insulator 418 be substantially aligned with the top surface of the insulator 419. Furthermore, the level of the top surface of the insulator 409 is lower than the levels of the top surfaces of the insulators 418 and 419.

Hereinafter, the oxide 406 a and the oxide 406 b may be collectively referred to as an oxide 406. Although the transistor 1000 has a structure in which the oxide 406 a and the oxide 406 b are stacked, one embodiment of the present invention is not limited to this structure. For example, a structure in which only the oxide 406 b is provided may be employed. Furthermore, the conductor 404 a and the conductor 404 b may be collectively referred to as a conductor 404. Although the transistor 1000 has a structure in which the conductor 404 a and the conductor 404 b are stacked, one embodiment of the present invention is not limited to this structure. For example, a structure in which only the conductor 404 b is provided may be employed.

The transistor 1000 may have a structure in which an insulator 432 is provided over the substrate. The transistor 1000 may include an insulator 430 provided over the insulator 432, and a conductor 440 embedded in the insulator 430. An insulator 401 may be provided over the insulator 430, and an insulator 301 may be provided over the insulator 401. The transistor 1000 may include a conductor 310 embedded in the insulators 401 and 301. The conductor 310 is preferably provided to be in contact with the top surface of the conductor 440 and to overlap with the oxide 406 and the conductor 404. The transistor 1000 may further include an insulator 302 provided over the insulator 301 and the conductor 310, and an insulator 303 provided over the insulator 302, in which case the insulator 402 is provided over the insulator 303.

In the conductor 440, a conductor 440 a is formed in contact with an inner wall of an opening in the insulator 430, and a conductor 440 b is formed on the inner side. Here, the top surfaces of the conductors 440 a and 440 b can have substantially the same level as the top surface of the insulator 430. Although the transistor 1000 has a structure in which the conductor 440 a and the conductor 440 b are stacked, the structure of the present invention is not limited thereto. For example, only the conductor 440 b may be provided.

In the conductor 310, a conductor 310 a is formed in contact with an inner wall of an opening in the insulators 401 and 301, and a conductor 310 b is formed on the inner side. Thus, the conductor 310 a is preferably in contact with the conductor 440 b. Here, the top surfaces of the conductors 310 a and 310 b can have substantially the same level as the top surface of the insulator 301. Although the transistor 1000 has a structure in which the conductor 310 a and the conductor 310 b are stacked, the structure of the present invention is not limited thereto. For example, only the conductor 310 b may be provided.

The conductor 404 is positioned to extend in the channel width direction. The conductor 404 can serve as a top gate, and the conductor 310 can serve as a back gate. The potential of the back gate may be the same as that of the top gate, the ground potential, or a given potential. By changing the potential of the back gate independently of the potential of the top gate, the threshold voltage of the transistor can be changed.

The conductor 440 extends in the channel width direction in a similar manner to the conductor 404, and serves as a wiring through which a potential is applied to the conductor 310 (i.e., the back gate). When the conductor 310 that is embedded in the insulators 401 and 301 is stacked over the conductor 440 serving as the wiring for the back gate, the insulators 401 and 301 and the like are positioned between the conductor 440 and the conductor 404, whereby the parasitic capacitance between the conductor 440 and the conductor 404 can be reduced and the withstand voltage can be increased. The reduction in the parasitic capacitance between the conductor 440 and the conductor 404 can improve the switching speed of the transistor, so that the transistor can have high frequency characteristics. The increase in the withstand voltage between the conductor 440 and the conductor 404 can improve the reliability of the transistor 1000. Thus, the thicknesses of the insulator 401 and the insulator 301 are preferably large. Note that the extending direction of the conductor 440 is not limited to this example. The conductor 440 may extend in the channel length direction of the transistor 1000, for example.

Each of the conductor 310 a and the conductor 440 a is preferably formed using a conductive material having a function of inhibiting the penetration of impurities such as water or hydrogen (a conductive material that is unlikely to transmit impurities). Each of the conductor 310 a and the conductor 440 a is a single layer or a stacked layer, preferably formed using tantalum, tantalum nitride, ruthenium, ruthenium oxide, or the like. Accordingly, diffusion of impurities such as hydrogen or water from a layer under the insulator 432 into an upper layer through the conductors 440 and 310 can be inhibited. Note that it is preferable that each of the conductor 310 a and the conductor 440 a have a function of inhibiting the penetration of at least one of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, a nitrogen molecule, a nitrogen oxide molecule (e.g., N₂O, NO, or NO₂), and a copper atom, and oxygen (e.g., an oxygen atom or an oxygen molecule). Furthermore, hereinafter, the same applies to the description of a conductive material that is capable of inhibiting the penetration of impurities. When each of the conductor 310 a and the conductor 440 a has a function of inhibiting the penetration of oxygen, the conductor 310 b and the conductor 440 b can be prevented from being oxidized and reduced in conductivity.

The conductor 310 b is preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. Although not illustrated, the conductor 310 b may have a stacked-layer structure. For example, titanium, titanium nitride, and the above conductive material may be stacked as the conductor 310 b.

The conductor 440 b, which serves as a wiring, is preferably formed using a conductor having a higher conductivity than the conductor 310 b; a conductive material containing copper or aluminum as its main component can be used, for example. Although not shown, the conductor 440 b may have a stacked layer structure, and for example, stacked layers of titanium, titanium nitride, and the above-described conductive material may be formed.

The insulator 432 and the insulator 401 can each serve as a barrier insulating film for preventing impurities such as water or hydrogen from entering the transistor from a lower layer. The insulator 432 and the insulator 401 are each preferably formed using an insulating material having a function of inhibiting the penetration of impurities such as water or hydrogen. For example, it is preferable that the insulator 432 be formed using aluminum oxide and the insulator 401 be formed using silicon nitride. In this way, impurities such as hydrogen or water can be inhibited from diffusing to layers above the insulators 432 and 401. Note that it is preferable that the insulators 432 and 401 each have a function of inhibiting the penetration of at least one of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, a nitrogen molecule, a nitrogen oxide molecule (e.g., N₂O, NO, and NO₂), and a copper atom. Furthermore, in the following description, the same applies to an insulating material having a function of inhibiting the penetration of impurities.

Furthermore, each of the insulator 432 and the insulator 401 is preferably formed using an insulating material that is capable of inhibiting the penetration of oxygen (e.g., an oxygen atom or an oxygen molecule). Thus, oxygen contained in the insulator 402 or the like can be prevented from being diffused to lower layers.

Furthermore, with the structure in which the conductor 310 is stacked over the conductor 440, the insulator 401 can be provided between the conductor 440 and the conductor 310. Here, even when a metal that is easily diffused, such as copper, is used as the conductor 440 b, silicon nitride or the like provided as the insulator 401 can prevent diffusion of the metal to a layer positioned above the insulator 401.

The insulator 303 is preferably formed using an insulating material that is capable of inhibiting the penetration of oxygen and impurities such as water or hydrogen, and is preferably formed using aluminum oxide or hafnium oxide, for example. Accordingly, diffusion of impurities such as hydrogen or water from a layer under the insulator 303 into a layer over the insulator 303 can be inhibited. Furthermore, oxygen contained in the insulator 402 or the like can be prevented from being diffused to lower layers.

Furthermore, the concentration of impurities such as water, hydrogen, or nitrogen oxide in the insulator 402 is preferably lowered. The amount of hydrogen released from the insulator 402, which is converted into hydrogen molecules per unit area of the insulator 402, is less than or equal to 2×10¹⁵ molecules/cm², preferably less than or equal to 1×10¹⁵ molecules/cm², further preferably less than or equal to 5×10¹⁴ molecules/cm² in thermal desorption spectroscopy (TDS) in the range of 50° C. to 500° C., for example. The insulator 402 is preferably formed using an insulator from which oxygen is released by heating.

The insulator 412 can serve as a first gate insulating film. The insulators 302, 303, and 402 can serve as second gate insulating films. Although the transistor 1000 has a structure in which the insulators 302, 303, and 402 are stacked, one embodiment of the present invention is not limited to this structure. For example, a structure in which any two selected from the insulators 302, 303, and 402 are stacked or a structure in which any one selected from the insulators 302, 303, and 402 is used may be employed.

The oxide 406 is preferably formed using a metal oxide serving as an oxide semiconductor (hereinafter, such a metal oxide may also be referred to as an oxide semiconductor). The metal oxide to be used preferably has an energy gap greater than or equal to 2 eV, further preferably greater than or equal to 2.5 eV. With the use of a metal oxide having such a wide energy gap, the off-state current of the transistor can be reduced.

A transistor formed using an oxide semiconductor has an extremely low leakage current in an off state; thus, a semiconductor device with low power consumption can be provided. An oxide semiconductor can be deposited by a sputtering method or the like, and thus can be used in a transistor included in a highly integrated semiconductor device.

An oxide semiconductor preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the like may be contained.

Here, a case where the oxide semiconductor is an In-M-Zn oxide containing indium, an element M, and zinc is considered. The element M is aluminum, gallium, yttrium, tin, or the like. Other elements that can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like. Note that two or more of the above elements may be used in combination as the element M.

Note that in this specification and the like, a metal oxide including nitrogen is also called a metal oxide in some cases. Moreover, a metal oxide including nitrogen may be called a metal oxynitride.

Here, the atomic ratio of the element M to the constituent elements of the metal oxide used as the oxide 406 a is preferably greater than that of the metal oxide used as the oxide 406 b. Moreover, the atomic ratio of the element M to In in the metal oxide used as the oxide 406 a is preferably greater than that in the metal oxide used as the oxide 406 b. In addition, the atomic ratio of In to the element M in the metal oxide used as the oxide 406 b is preferably greater than that in the metal oxide used as the oxide 406 a.

When using the above metal oxide as the oxide 406 a, it is preferable that the conduction band minimum of the oxide 406 a be higher than the conduction band minimum of the region of the oxide 406 b where the conduction band minimum is low. In other words, the electron affinity of the oxide 406 a is preferably smaller than the electron affinity of the region of the oxide 406 b where the conduction band minimum is low.

Here, the energy level of the conduction band minimum is gradually varied in the oxides 406 a and 406 b. In other words, the energy level of the conduction band minimum is continuously varied or continuously connected. To vary the energy level gradually, the density of defect states in a mixed layer formed at the interface between the oxides 406 a and 406 b is decreased.

Specifically, when the oxides 406 a and 406 b contain the same element (as a main component) in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, in the case where the oxide 406 b is an In—Ga—Zn oxide, it is preferable to use an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like as the oxide 406 a.

At this time, a narrow-gap portion formed in the oxide 406 b serves as a main carrier path. Since the density of defect states at the interface between the oxides 406 a and 406 b can be decreased, the influence of interface scattering on carrier conduction is small, and high on-state current can be obtained.

The oxide 406 includes a region 426 a, a region 426 b, and a region 426 c. As illustrated in FIG. 1B, the region 426 a is sandwiched between the region 426 b and the region 426 c. The regions 426 b and 426 c are reduced in resistance through formation of the insulator 409, and are high in conductivity than the region 426 a. Impurity elements such as hydrogen or nitrogen, which are contained in an atmosphere where the insulator 409 is formed, are added to the regions 426 b and 426 c. Accordingly, oxygen vacancies are formed because of the added impurity elements, and the impurity elements enter the oxygen vacancies, thereby increasing the carrier density and reducing resistance.

Thus, it is preferable that the concentration of at least one of hydrogen and nitrogen be higher in the regions 426 b and 426 c than in the region 426 a. The concentration of hydrogen or nitrogen is measured by secondary ion mass spectrometry (SIMS) or the like. As the concentration of hydrogen or nitrogen in the region 426 a, the concentration of hydrogen or nitrogen near the center of a region of the oxide 406 b that overlaps with the insulator 412 (e.g., a portion of the oxide 406 b, which is substantially equally away from the left and right side surfaces of the insulator 412 in the channel length direction) is measured.

The regions 426 b and 426 c are reduced in resistance when an element forming an oxygen vacancy or an element bonded to an oxygen vacancy is added thereto. Typical examples of such an element are hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, and a rare gas element. Typical examples of the rare gas element are helium, neon, argon, krypton, and xenon. Accordingly, the regions 426 b and 426 c are made to include one or more of the above elements.

It is preferable in the oxide 406 a that the atomic ratio of In to the element M in the regions 426 b and 426 c be substantially the same as that in the oxide 406 b. In other words, in the oxide 406 a, the atomic ratio of In to the element M in the regions 426 b and 426 c is preferably greater than that in the region 426 a. Here, when the indium content in the oxide 406 is increased, the carrier density is increased and the resistance can be decreased. With this structure, even when the thickness of the oxide 406 b is small and electric resistance of the oxide 406 b is high in the manufacturing process of the transistor 1000, the regions 426 b and 426 c in the oxide 406 can serve as source and drain regions owing to the sufficiently reduced resistance of the oxide 406 a in the regions 426 b and 426 c.

FIG. 4A is an enlarged view of the region 426 a and its vicinity illustrated in FIG. 1B. As illustrated in FIG. 4A, the regions 426 b and 426 c are formed in regions of the oxide 406 that overlap with at least the insulator 409. One of the regions 426 b and 426 c of the oxide 406 b can serve as a source region, and the other can serve as a drain region. The region 426 a of the oxide 406 b can serve as a channel formation region.

Although the regions 426 a, 426 b, and 426 c are formed in the oxides 406 b and 406 a in FIG. 1B and FIG. 4A, it is acceptable as long as the regions 426 a, 426 b, and 426 c are formed at least in the oxide 406 b. Although the boundary between the regions 426 a and 426 b and the boundary between the regions 426 a and 426 c are substantially perpendicular to the top surface of the oxide 406 in FIG. 1B and the like, one embodiment of the present invention is not limited thereto. For example, the region 426 b is shaped such that it recedes to the A1 (in FIG. 1B) side near the bottom surface of the oxide 406 a and the region 426 c is shaped such that it recedes to the A2 (in FIG. 1B) side near the bottom surface of the oxide 406 a, in some cases.

In the transistor 1000, as illustrated in FIG. 4A, the regions 426 b and 426 c each extend from an end portion of the oxide 406 to a region of the oxide 406 that overlaps with an end portion of the insulator 412. In that case, portions of the regions 426 b and 426 c that overlap with the conductor 404 serve as what we call overlap regions (also referred to as Lov regions). With the Lov regions, no high-resistance region is formed between the channel formation region and the source or drain region of the oxide 406; accordingly, the on-state current and mobility of the transistor can be increased.

However, the semiconductor device described in this embodiment is not limited to the above-described structure. For example, as illustrated in FIG. 4B, each of the regions 426 b and 426 c may extend from an end portion of the oxide 406 to a region of the oxide 406 that overlaps with the insulator 418. The structure illustrated in FIG. 4B can be rephrased as the structure in which the width of the conductor 404 in the channel length direction is substantially the same as the width of the region 426 a. Because a high-resistance region is not formed between the source region and the drain region in the structure illustrated in FIG. 4B, the on-state current of the transistor can be increased. Since the gate does not overlap with the source and drain regions in the channel length direction in the structure illustrated in FIG. 4B, formation of unnecessary capacitance can be suppressed.

By appropriately selecting the areas of the regions 426 b and 426 c in the above manners, a transistor having desired electrical characteristics can be easily provided in accordance with the circuit design.

The insulator 412 is preferably provided in contact with the top surface of the oxide 406 b. The insulator 412 is preferably formed using an insulator from which oxygen is released by heating. When the insulator 412 formed using such a material is provided in contact with the top surface of the oxide 406 b, oxygen can be supplied to the oxide 406 b effectively. Furthermore, the concentration of impurities such as water or hydrogen in the insulator 412 is preferably lowered as in the insulator 402. The thickness of the insulator 412 is preferably 1 nm to 20 nm inclusive (e.g., approximately 10 nm).

The insulator 412 preferably contains oxygen. The amount of released oxygen molecules per unit area of the insulator 412 is more than or equal to 1×10¹⁴ molecules/cm², preferably more than or equal to 2×10¹⁴ molecules/cm², further preferably more than or equal to 4×10¹⁴ molecules/cm² in thermal desorption spectroscopy (TDS) in the range of the film surface temperatures from 100° C. to 700° C. inclusive or from 100° C. to 500° C. inclusive, for example.

The insulator 412, the conductor 404, and the insulator 419 have regions overlapping with the oxide 406 b. In addition, it is preferable that side surfaces of the insulator 412, the conductor 404 a, the conductor 404 b, and the insulator 419 be substantially aligned with each other.

The conductor 404 a is preferably formed using a conductive oxide. For example, the metal oxide that can be used as the oxide 406 a or the oxide 406 b can be used for the conductor 404 a. In particular, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=4:2:3 to 4:2:4.1 or in the neighborhood thereof, which has high conductivity, is preferably used. When the conductor 404 a is formed using such a material, oxygen can be prevented from entering the conductor 404 b, and an increase in electric resistance value of the conductor 404 b due to oxidation can be prevented.

In addition, by depositing such a conductive oxide by sputtering, oxygen can be added to the insulator 412, which makes it possible to supply oxygen to the oxide 406 b. Thus, oxygen vacancies in the region 426 a of the oxide 406 can be reduced.

The conductor 404 b can be formed using a metal such as tungsten, for example. It is also possible to use, as the conductor 404 b, a conductor that can add impurities such as nitrogen to the conductor 404 a to improve the conductivity of the conductor 404 a. For example, titanium nitride or the like is preferably used for the conductor 404 b. Alternatively, the conductor 404 b may be a stack including a metal nitride such as titanium nitride and a metal such as tungsten thereover.

Here, the conductor 404 functioning as a gate electrode is provided to cover the top surface of the region 426 a and its periphery and the side surface, which is in the channel width direction, of the oxide 406 b with the insulator 412 interposed therebetween. Thus, the electric field of the conductor 404 functioning as a gate electrode can electrically surround the top surface of the region 426 a and its periphery and the side surface, which is in the channel width direction, of the oxide 406 b. The structure of the transistor in which the channel formation region is electrically surrounded by the electric field of the conductor 404 is referred to as a surrounded channel (s-channel) structure. Thus, a channel can be formed in the top surface of the region 426 a and its periphery and the side surface, which is in the channel width direction, of the oxide 406 b; therefore, a large amount of current can flow between the source and the drain, and a current in an on state (on-state current) can be large. Moreover, since the top surface of the region 426 a and its periphery and the side surface, which is in the channel width direction, of the oxide 406 b are surrounded by the electric field of the conductor 404, a leakage current in an off state (off-state current) can be small.

The insulator 419 is preferably provided over the conductor 404 b. In addition, it is preferable that the position of a side surface of the insulator 412 be substantially the same as the positions of side surfaces of the insulator 419, the conductor 404 a, and the conductor 404 b when seen perpendicularly to the substrate from the above. The insulator 419 is preferably formed by an atomic layer deposition (ALD) method. In that case, the insulator 419 can be formed with a thickness of approximately 1 nm to 20 nm inclusive, preferably approximately 5 nm to 10 nm inclusive. Similarly to the insulator 418, the insulator 419 is preferably formed using an insulating material having a function of inhibiting the penetration of oxygen and impurities such as water or hydrogen, and is preferably formed using aluminum oxide or hafnium oxide, for example.

When the insulator 419 is provided as described above, the insulator 419 and the insulator 418 each of which has a function of inhibiting the penetration of oxygen and impurities such as water or hydrogen can cover the top and side surfaces of the conductor 404. This can prevent entry of impurities such as water or hydrogen into the oxide 406 through the conductor 404. Thus, the insulator 418 and the insulator 419 have a function of a gate cap that protects the gate.

The insulator 418 is provided in contact with the side surfaces of the insulator 412, the conductor 404, and the insulator 419. Furthermore, it is preferable that the top surface of the insulator 418 be substantially aligned with the top surface of the insulator 419. The insulator 418 is preferably deposited by an ALD method, in which case the thickness of the insulator 418 can be approximately 3 nm to 20 nm inclusive, preferably approximately 5 nm to 10 nm (e.g., 5 nm). Note that a precursor used in the ALD method sometimes contains impurities such as carbon. Thus, the insulator 418 may contain impurities such as carbon. In the case where the insulator 401 is formed by sputtering and the insulator 418 is formed by an ALD method, for example, the insulator 418 may contain more impurities such as carbon than the insulator 401 even when the insulators 418 and 401 are both formed using aluminum oxide. Note that impurities can be quantified by X-ray photoelectron spectroscopy (XPS).

The regions 426 b and 426 c of the oxide 406 are formed because of the impurity elements added in the formation of the insulator 409. In the case where a transistor is miniaturized to have a channel length of approximately 10 nm to 30 nm inclusive, impurity elements contained in a source region or a drain region may diffuse and the source region and the drain region may be electrically connected to each other. By contrast, when the insulator 418 is formed as described in this embodiment, the distance between the region 426 b of the oxide 406 and the region 426 c of the oxide 406 can be longer; thus, the source region and the drain region can be prevented from being electrically connected to each other. Moreover, the insulator 418 formed by an ALD method can have a thickness substantially equal to or less than a miniaturized channel length, which can prevent the distance between the source and drain regions from being longer than necessary and the resistance from increasing.

Here, the insulators 418 and 410 are preferably formed using an insulating material that is capable of inhibiting the penetration of oxygen and impurities such as water or hydrogen, and is preferably formed using aluminum oxide or hafnium oxide, for example. In this manner, the insulator 418 can prevent oxygen in the insulator 412 from diffusing outward. In addition, impurities such as hydrogen or water can be prevented from entering the oxide 406 through the end portion of the insulator 412 or the like. Furthermore, the insulator 410 can suppress entry of impurities such as hydrogen or water from the insulator 420 and the like into the oxide 406.

The insulators 418 and 410 are formed by forming an insulating film 418A by an ALD method and then performing anisotropic etching. The etching is performed so that part of the insulating film in contact with the side surfaces of the insulator 412, the conductor 404, and the insulator 419 remains to be the insulator 418, and another part of the insulating film in contact with part of the side surface of the oxide 406 a and part of the side surface of the oxide 406 b remains to be the insulator 410. Thus, the insulators 418 and 410 having a small thickness as described above can be easily formed. At this time, even when the insulator 419 provided over the conductor 404 is partly removed by the anisotropic etching, the portion of the insulator 418 in contact with the insulator 412 and the conductor 404 can be left sufficiently.

The insulators 409 and 420 are formed by forming an insulating film 409A to cover the insulator 419, the insulator 418, the oxide 406, and the insulator 402 and then performing anisotropic etching. With the use of the insulating film 409A, impurities such as hydrogen or nitrogen are added to the oxide 406 to form the regions 426 b and 426 c. Thus, the insulators 409 and 420 preferably contain at least one of hydrogen and nitrogen. By performing anisotropic etching on the insulating film 409A, the insulator 409 is formed in contact with the side surface of the insulator 418, and the insulator 420 is formed in contact with part of the side surface of the oxide 406 and the side surface of the insulator 410.

The insulating film 409A is preferably formed using an insulating material that is capable of inhibiting the penetration of oxygen and impurities such as water or hydrogen. For example, the insulating film 409A is preferably formed using silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum nitride, or aluminum nitride oxide.

An insulator 415 is preferably provided to cover the insulator 402, the insulator 420, the oxide 406, the insulator 409, the insulator 418, and the insulator 419. The concentration of impurities such as water or hydrogen in the insulator 415 is preferably lowered as in the insulator 402 and the like. An insulator that is similar to the insulator 432 may be provided over the insulator 415.

Openings are formed in the insulator 415 so that inner walls of the openings in the insulator 415 are in contact with side surfaces of the insulator 409. In order to form such openings, it is preferable that the etching rate of the insulator 409 be extremely lower than that of the insulator 415 at the time of forming the openings in the insulator 415. When the etching rate of the insulator 409 is set to 1, the etching rate of the insulator 415 is preferably set to 5 or more, further preferably 10 or more.

Here, a conductor 450 a is formed in contact with the inner wall of one opening in the insulator 415, and a conductor 451 a is formed on the inner side. The region 426 b of the oxide 406 is positioned in at least part of a bottom portion of the opening, and the conductor 450 a is in contact with the region 426 b. Similarly, a conductor 450 b is formed in contact with an inner wall of the other opening in the insulator 415, and a conductor 451 b is formed on the inner side. The region 426 c of the oxide 406 is positioned in at least part of a bottom portion of the opening, and the conductor 450 b is in contact with the region 426 c.

The conductors 450 a and 451 a and the conductors 450 b and 451 b are preferably provided to face each other with the conductor 404 positioned therebetween. With such a structure, the distance between the conductors 450 a and 451 a and the conductors 450 b and 451 b can be small; thus, a miniaturized semiconductor device can be fabricated, which is preferable.

Here, FIG. 6 is a cross-sectional view of a portion along the dashed-dotted line A5-A6 in FIG. 1A. Although the cross-sectional view in FIG. 6 illustrates the conductors 450 b and 451 b, the conductors 450 a and 451 a have a similar structure.

As illustrated in FIGS. 1B and 1C and FIG. 6, upper corners of the oxide 406 are rounded in a cross section. The conductor 450 b is in contact with at least the top surface of the oxide 406 and is preferably in contact with the rounded corners of the oxide 406. Specifically, as illustrated in FIG. 6, the conductor 450 b is preferably in contact with part of one or both of rounded corners (the rounded corners on the A5 side and the A6 side) in the channel width direction of the oxide 406. As illustrated in FIG. 1B, the conductor 450 b may be in contact with part of the rounded corner on the A2 side in the channel length direction of the oxide 406. Thus, when the structure in which the conductor 450 b is in contact with part of the rounded corner of the oxide 406 in addition to the top surface of the oxide 406 is employed, the contact area between the conductor 450 b and the oxide 406 can be increased without an increase in the area of the top surface of the contact portion, so that the contact resistance between the conductor 450 b and the oxide 406 can be reduced. Accordingly, miniaturization of the source electrode and the drain electrode of the transistor can be achieved and, in addition, the on-state current can be increased. Note that the same applies to the conductor 450 a and the conductor 451 a. The curvature radius of the rounded corner of the oxide 406 is preferably greater than or equal to one-fourth of the thickness of the oxide 406 and less than or equal to the thickness of the oxide 406.

As illustrated in FIG. 1B and FIGS. 4A and 4B, the conductor 450 a is in contact with the region 426 b serving as one of a source region and a drain region of the transistor 1000, and the conductor 450 b is in contact with the region 426 c serving as the other of the source region and the drain region of the transistor 1000. Thus, the conductors 450 a and 451 a can serve as one of a source electrode and a drain electrode, and the conductors 450 b and 451 b can serve as the other of the source electrode and the drain electrode. The insulator 409 is provided, the insulator 415 is provided over the insulator 409, the contact holes are formed in the insulator 415 to expose the insulator 409, and the conductors 450 a and 450 b functioning as a source electrode and a drain electrode are formed in the contact holes, whereby the distance between a region in which the conductor 450 a and the oxide 406 are in contact with each other and the region 426 a, which is a channel formation region, can be small; thus, the parasitic resistance of the region 426 b can be small. Similarly, the distance between a region in which the conductor 450 b and the oxide 406 are in contact with each other and the region 426 a, which is a channel formation region, can be small; thus, the parasitic resistance of the region 426 c can be small. Accordingly, the resistance of the region 426 b and the region 426 c, the contact resistance between the conductor 450 a and the region 426 b, and the contact resistance between the conductor 450 b and the region 426 c are reduced, leading to a large on-state current of the transistor 1000.

FIGS. 5A and 5B are enlarged views of the channel formation region in FIGS. 1A to 1C. FIG. 5A illustrates a cross section in the channel length direction. FIG. 5B illustrates a cross section in the channel width direction. As illustrated in FIGS. 5A and 5B, with the insulators 418 and 409 functioning as dielectrics, parasitic capacitance C_(sw1) is formed between the conductor 404 and the conductors 450 a and 451 a, and similarly, parasitic capacitance C_(sw2) is formed between the conductor 404 and the conductors 450 b and 451 b. Thus, parasitic capacitance C_(sw) of the transistor 1000 is the sum of the parasitic capacitance C_(sw1) and the parasitic capacitance C_(sw2). Since the structure is symmetrical as illustrated in FIG. 5A, the parasitic capacitance C_(sw1) and the parasitic capacitance C_(sw2) are approximately equal to each other. The parasitic capacitance C_(sw) may affect the operation speed of the transistor 1000. Thus, the parasitic capacitance C_(sw) is preferably smaller than gate capacitance C_(gi) formed between the conductor 404 and the region 426 a with the insulator 412 functioning as a dielectric.

When the total thickness of the insulators 418 and 409 in the channel length direction is t_(sw) and the thickness of the insulator 412 is t_(gi), preferable relation between t_(sw) and t_(gi) is described below with reference to formulae. The parasitic capacitance C_(sw) can be obtained by Formula 1 when the permittivity of the insulators 418 and 409 is ε_(sw), the thickness of the conductor 404 is t_(gate), and the line width of the conductor 404 in the channel width direction, that is, an apparent channel width is W.

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \mspace{625mu}} & \; \\ {C_{sw} = \frac{2\; ɛ_{sw}t_{gate}w}{t_{sw}}} & (1) \end{matrix}$

The gate capacitance C_(gi) can be obtained by Formula 2 when the permittivity of the insulator 412 is ε_(gi), the width of the conductor 404 in the channel length direction is L. Here, as W in Formula 2, the apparent channel width is used as in Formula 1.

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \mspace{625mu}} & \; \\ {C_{gi} = \frac{ɛ_{gi}{Lw}}{t_{gi}}} & (2) \end{matrix}$

Here, when the parasitic capacitance C_(sw) is less than or equal to the gate capacitance C_(gi), and t_(sw), which is the total thickness of the insulators 418 and 409, is smaller than or equal to L, which is the width of the conductor 404 in the channel length direction, an inequality shown in Formula 3 can be satisfied. Thus, t_(sw) and t_(gi) preferably satisfy Formula 3.

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \mspace{625mu}} & \; \\ {\frac{2\; ɛ_{sw}t_{gate}t_{gi}}{ɛ_{gi}L} \leqq t_{sw} \leqq L} & (3) \end{matrix}$

Next, as a specific example, a case where t_(gate) is equal to L, and, t_(gi) is one-sixth of L is described. When these are substituted into Formula 3, Formula 4 and Formula 5 are obtained.

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack \mspace{625mu}} & \; \\ {{\frac{2\; ɛ_{sw}}{ɛ_{gi}}t_{gi}} \leqq t_{sw} \leqq L} & (4) \\ {\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack \mspace{625mu}} & \; \\ {{2\frac{ɛ_{sw}}{ɛ_{gi}}} \leqq \frac{t_{sw}}{t_{gi}} \leqq 6} & (5) \end{matrix}$

Next, each of the thicknesses of the insulators 409, 418, and 412 is represented by the thickness of a silicon oxide film (an equivalent oxide thickness (EOT)). If the equivalent thickness of t_(sw) is represented by T_(sw) and the equivalent thickness of t_(gi) is represented by T_(gi), ε_(sw) and ε_(gi) in Formula 5 are eliminated, so that Formula 6 is obtained. That is, T_(sw) is preferably 2 times to 6 times T_(gi). In other words, T_(gi) is preferably ⅙ to ½ of T_(sw).

$\begin{matrix} {\left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack \mspace{625mu}} & \; \\ {2 \leqq \frac{T_{sw}}{T_{gi}} \leqq 6} & (6) \end{matrix}$

The conductor 450 a and the conductor 450 b are each preferably formed using a conductive material having a function of inhibiting the penetration of impurities such as water or hydrogen, like the conductor 310 a or the like. For example, tantalum, tantalum nitride, titanium, titanium nitride, ruthenium, ruthenium oxide, or the like is preferably used, and a single layer or stacked layers may be used. This can prevent entry of impurities such as hydrogen or water from a layer positioned over the insulator 415 to the oxide 406 through the conductor 451 a and the conductor 451 b. Furthermore, the conductor 451 a and the conductor 451 b are each preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. Although not shown, the conductor 451 a and the conductor 451 b may have a stacked layer structure, and for example, stacked layers of titanium, titanium nitride, and the above-described conductive material may be formed.

Note that the top surfaces of the conductor 450 a, the conductor 451 a, the conductor 450 b, and the conductor 451 b can be on approximately the same level. Furthermore, although the transistor 1000 in which the conductor 450 a and the conductor 451 a are stacked and the conductor 450 b and the conductor 451 b are stacked is described here, one embodiment of the present invention is not limited to this example. For example, a structure in which only the conductor 451 a and the conductor 451 b are provided may be employed.

It is preferable that a conductor 452 a be provided in contact with the top surface of the conductor 451 a and a conductor 452 b be provided in contact with the top surface of the conductor 451 b. The conductor 452 a and the conductor 452 b are each preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. Although not shown, the conductor 452 a and the conductor 452 b may each have a stacked layer structure, and for example, stacked layers of titanium, titanium nitride, and the above-described conductive material may be formed. Note that the conductor 452 a and the conductor 452 b may be embedded in the openings provided in the insulators, in a similar manner to the conductor 440 or the like.

Next, components of the transistor 1000 are described.

<Substrate>

As the substrate over which the transistor 1000 is formed, for example, an insulator substrate, a semiconductor substrate, or a conductor substrate may be used. Examples of the insulator substrate include a glass substrate, a quartz substrate, a sapphire substrate, a stabilized zirconia substrate (e.g., an yttria-stabilized zirconia substrate), and a resin substrate. Examples of the semiconductor substrate include a semiconductor substrate of silicon, germanium, or the like, and a compound semiconductor substrate of silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, or gallium oxide. Examples of the semiconductor substrate also include a semiconductor substrate in which an insulator region is provided in the above semiconductor substrate, e.g., a silicon on insulator (SOI) substrate. Examples of the conductor substrate include a graphite substrate, a metal substrate, an alloy substrate, and a conductive resin substrate. Examples of the conductor substrate also include a substrate containing a metal nitride and a substrate containing a metal oxide. An insulator substrate provided with a conductor or a semiconductor, a semiconductor substrate provided with a conductor or an insulator, a conductor substrate provided with a semiconductor or an insulator, or the like can also be used. Alternatively, any of these substrates over which an element is provided may be used. As the element provided over the substrate, a capacitor, a resistor, a switching element, a light-emitting element, a memory element, or the like can be given.

Alternatively, a flexible substrate may be used as the substrate. As a method for providing a transistor over a flexible substrate, there is a method in which the transistor is formed over a non-flexible substrate and then the transistor is separated and transferred to the substrate that is a flexible substrate. In that case, a separation layer is preferably provided between the non-flexible substrate and the transistor. As the substrate, a film, a foil, or a sheet containing a fiber may be used. The substrate may have elasticity. The substrate may have a property of returning to its original shape when bending or pulling is stopped. Alternatively, the substrate may have a property of not returning to its original shape. The substrate includes a region having a thickness of, for example, greater than or equal to 5 μm and less than or equal to 700 μm, preferably greater than or equal to 10 μm and less than or equal to 500 μm, and further preferably greater than or equal to 15 μm and less than or equal to 300 μm. When the substrate has a small thickness, the weight of the semiconductor device including the transistor can be reduced. When the substrate has a small thickness, even in the case of using glass or the like, the substrate may have elasticity or a property of returning to its original shape when bending or pulling is stopped. Therefore, an impact applied to the semiconductor device over the substrate, which is caused by dropping or the like, can be reduced. That is, a durable semiconductor device can be provided.

For the substrate that is a flexible substrate, metal, an alloy, resin, glass, or fiber thereof can be used, for example. The flexible substrate preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10⁻³/K, lower than or equal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, and acrylic. In particular, aramid is suitably used for the flexible substrate because of its low coefficient of linear expansion.

<Insulator>

The insulator can be an oxide, nitride, oxynitride, nitride oxide, metal oxide, metal oxynitride, metal nitride oxide, or the like having an insulating property.

Note that when the transistor is surrounded by an insulator that has a function of inhibiting the penetration of oxygen and impurities such as hydrogen, the electrical characteristics of the transistor can be stabilized. For example, an insulator that has a function of inhibiting the penetration of oxygen and impurities such as hydrogen is used for each of the insulators 303, 401, and 432.

The insulator that has a function of inhibiting the penetration of oxygen and impurities such as hydrogen can have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum.

Furthermore, for example, the insulators 303, 401, and 432 may each be formed using a metal oxide such as aluminum oxide, magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide; silicon nitride oxide; or silicon nitride. Note that the insulators 303, 401, and 432 preferably contain aluminum oxide, hafnium oxide, or the like.

The insulators 430, 301, 302, 402, and 412 may each be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. For example, the insulators 430, 301, 302, 402, and 412 each preferably contain silicon oxide, silicon oxynitride, or silicon nitride.

It is preferable that the insulator 302, the insulator 303, the insulator 402, and/or the insulator 412 be formed using an insulator with a high dielectric constant. For example, it is preferable that the insulator 302, the insulator 303, the insulator 402, and/or the insulator 412 contain gallium oxide, hafnium oxide, zirconium oxide, an oxide containing aluminum and hafnium, an oxynitride containing aluminum and hafnium, an oxide containing silicon and hafnium, an oxynitride containing silicon and hafnium, or a nitride containing silicon and hafnium. Alternatively, it is preferable that the insulator 302, the insulator 303, the insulator 402, and/or the insulator 412 have a stacked-layer structure of silicon oxide or silicon oxynitride and an insulator with a high dielectric constant. Because silicon oxide and silicon oxynitride have thermal stability, a combination of silicon oxide or silicon oxynitride with an insulator with a high dielectric constant allows the stacked-layer structure to be thermally stable and have a high dielectric constant. For example, when aluminum oxide, gallium oxide, or hafnium oxide is positioned in contact with the oxide 406 in each of the insulators 402 and 412, silicon contained in silicon oxide or silicon oxynitride can be prevented from entering the oxide 406. Furthermore, for example, when silicon oxide or silicon oxynitride is in contact with the oxide 406 in each of the insulators 402 and 412, trap centers might be formed at the interface between aluminum oxide, gallium oxide, or hafnium oxide and silicon oxide or silicon oxynitride. The trap centers can shift the threshold voltage of the transistor in the positive direction by trapping electrons, in some cases.

Each of the insulators 430, 301, and 415 preferably includes an insulator with a low dielectric constant. For example, each of the insulators 430, 301, and 415 preferably contains silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, a resin, or the like. Alternatively, each of the insulators 430, 301, and 415 preferably has a stacked-layer structure of a resin and one of the following materials: silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, and porous silicon oxide. Because silicon oxide and silicon oxynitride have thermal stability, a combination of silicon oxide or silicon oxynitride with a resin allows the stacked-layer structure to be thermally stable and have a low dielectric constant. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, and acrylic.

For each of the insulators 418 and 419, an insulator having a function of inhibiting the penetration of oxygen and impurities such as hydrogen is used. For each of the insulators 418 and 419, for example, a metal oxide such as aluminum oxide, hafnium oxide, magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, or tantalum oxide; silicon nitride oxide; or silicon nitride can be used.

<Conductor>

The conductors 404 a, 404 b, 310 a, 310 b, 450 a, 450 b, 451 a, 451 b, 452 a, and 452 b can be formed using a material containing one or more metal elements selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium, ruthenium, and the like. Alternatively, a semiconductor having a high electric conductivity typified by polycrystalline silicon containing an impurity element such as phosphorus, or silicide such as nickel silicide may be used.

For the above-described conductors, especially for the conductors 404 a, 310 a, 450 a, and 450 b, a conductive material containing oxygen and a metal element included in a metal oxide that can be used for the oxide 406 may be used. A conductive material containing the above-described metal element and nitrogen may be used. For example, a conductive material containing nitrogen such as titanium nitride or tantalum nitride may be used. An indium tin oxide, an indium oxide containing tungsten oxide, an indium zinc oxide containing tungsten oxide, an indium oxide containing titanium oxide, an indium tin oxide containing titanium oxide, an indium zinc oxide, or an indium tin oxide to which silicon is added may be used. An indium gallium zinc oxide containing nitrogen may be used. With the use of such a material, hydrogen contained in the oxide 406 can be captured in some cases. Alternatively, hydrogen entering from an external insulator or the like can be captured in some cases.

A stack including a plurality of conductive layers formed using the above materials may be used. For example, a stacked-layer structure formed using a combination of a material containing the above-described metal element and a conductive material containing oxygen may be used. Alternatively, a stacked-layer structure formed using a combination of a material containing the above-described metal element and a conductive material containing nitrogen may be used. Alternatively, a stacked-layer structure formed using a combination of a material containing the above-described metal element, a conductive material containing oxygen, and a conductive material including nitrogen may be used.

When the oxide is used for the channel formation region of the transistor, a stacked-layer structure formed using a material containing the above-described metal element and a conductive material containing oxygen is preferably used for the gate electrode. In that case, the conductive material containing oxygen is preferably formed on the channel formation region side. When the conductive material containing oxygen is formed on the channel formation region side, oxygen released from the conductive material is likely to be supplied to the channel formation region.

<Metal Oxide Applicable to Oxide 406>

The oxide 406 of the present invention is described below. For the oxide 406, a metal oxide functioning as an oxide semiconductor (hereinafter, the metal oxide is also referred to as an oxide semiconductor) is preferably used.

The oxide semiconductor preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more kinds of elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the like may be contained.

Here, a case where the oxide semiconductor is an In-M-Zn oxide containing indium, an element M, and zinc is considered. The element M is aluminum, gallium, yttrium, tin, or the like. Other elements that can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like. Note that two or more of the above elements may be used in combination as the element M.

Note that in this specification and the like, a metal oxide including nitrogen is also called a metal oxide in some cases. Moreover, a metal oxide including nitrogen may be called a metal oxynitride.

Here, the case where the metal oxide contains indium, the element M, and zinc is considered.

Preferred ranges of the atomic ratio of indium to the element M and zinc contained in the metal oxide that can be used for the oxides 406 a and 406 b are described with reference to FIGS. 21A to 21C. Note that the proportion of oxygen atoms is not shown in FIGS. 21A to 21C. The terms of the atomic ratio of indium to the element M and zinc contained in the metal oxide are denoted by [In], [M], and [Zn], respectively.

In FIGS. 21A to 21C, broken lines indicate a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):1 (−1≤α≤1), a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio [In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio [In]:[M]:[Zn] is (1+α): (1−α):5.

Furthermore, dashed-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn] is 5:1:β(β≥0), a line where the atomic ratio [In]:[M]:[Zn] is 2:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:1β, a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:3β, and a line where the atomic ratio [In]:[M]:[Zn] is 1:4:μ.

Furthermore, a metal oxide with the atomic ratio of [In]:[M]:[Zn]=0:2:1 or a neighborhood thereof in FIGS. 21A to 21C tends to have a spinel crystal structure.

A plurality of phases (e.g., two phases or three phases) exist in the metal oxide in some cases. For example, with an atomic ratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinel crystal structure and a layered crystal structure are likely to exist. In addition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0, two phases of a bixbyite crystal structure and a layered crystal structure are likely to exist. In the case where a plurality of phases exist in the metal oxide, a grain boundary might be formed between different crystal structures.

A region A in FIG. 21A represents an example of the preferred range of the atomic ratio of indium, the element M, and zinc contained in the metal oxide.

The metal oxide having a higher content of indium can have higher carrier mobility (electron mobility). Thus, a metal oxide having a high content of indium has higher carrier mobility than a metal oxide having a low content of indium.

By contrast, when the indium content and the zinc content in a metal oxide become lower, carrier mobility becomes lower. Thus, with an atomic ratio of [In]:[M]:[Zn]=0:1:0 and the vicinity thereof (e.g., a region C in FIG. 21C), insulation performance becomes better.

For example, the metal oxide used as the oxide 406 b preferably has an atomic ratio represented by the region A in FIG. 21A. The metal oxide with the atomic ratio has high carrier mobility. The atomic ratio of In to Ga and Zn of the metal oxide used as the oxide 406 b is 4:2:3 to 4:2:4.1 or in the neighborhood thereof, for example. By contrast, the metal oxide used as the oxide 406 a preferably has an atomic ratio represented by the region C in FIG. 21C. The metal oxide with the atomic ratio has relatively high insulating properties. The atomic ratio of In to Ga and Zn of the metal oxide used as the oxide 406 a is approximately 1:3:4.

A metal oxide having an atomic ratio in the region A, particularly in a region B in FIG. 21B, has high carrier mobility and high reliability and is excellent.

Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=4:2:3 to 4:2:4.1 and the vicinity thereof. The vicinity includes an atomic ratio of [In]:[M]:[Zn]=5:3:4. Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=5:1:6 and the vicinity thereof and an atomic ratio of [In]:[M]:[Zn]=5:1:7 and the vicinity thereof.

In the case where the metal oxide is formed of an In-M-Zn oxide, it is preferable to use a target including a polycrystalline In-M-Zn oxide as the sputtering target. Note that the atomic ratio of the formed metal oxide varies from the above atomic ratios of metal elements of the sputtering targets in a range of ±40%. For example, when a sputtering target with an atomic ratio of In:Ga:Zn=4:2:4.1 is used for forming the metal oxide, the atomic ratio of In to Ga and Zn in the formed metal oxide may be 4:2:3 or in the neighborhood of 4:2:3. When a sputtering target with an atomic ratio of In:Ga:Zn=5:1:7 is used for forming the metal oxide, the atomic ratio of In to Ga and Zn in the formed metal oxide may be 5:1:6 or in the neighborhood of 5:1:6.

Note that the property of a metal oxide is not uniquely determined by an atomic ratio. Even with the same atomic ratio, the property of a metal oxide might be different depending on a formation condition. For example, in the case where the metal oxide is deposited with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of the target is formed. In particular, [Zn] in the film might be smaller than [Zn] in the target depending on the substrate temperature in deposition. Thus, the illustrated regions each represent an atomic ratio with which a metal oxide tends to have specific characteristics, and boundaries of the regions A to C are not clear.

<Composition of Metal Oxide>

Described below is the composition of a cloud-aligned composite oxide semiconductor (CAC-OS) applicable to a transistor disclosed in one embodiment of the present invention.

In this specification and the like, “c-axis aligned crystal (CAAC)” or “cloud-aligned composite (CAC)” might be stated. Note that CAAC refers to an example of a crystal structure, and CAC refers to an example of a function or a material composition.

A CAC-OS or a CAC metal oxide has a conducting function in a part of the material and has an insulating function in another part of the material; as a whole, the CAC-OS or the CAC metal oxide has a function of a semiconductor. In the case where the CAC-OS or the CAC metal oxide is used in an active layer of a transistor, the conducting function is to allow electrons (or holes) serving as carriers to flow, and the insulating function is to not allow electrons serving as carriers to flow. By the complementary action of the conducting function and the insulating function, the CAC-OS or the CAC metal oxide can have a switching function (on/off function). In the CAC-OS or the CAC metal oxide, separation of the functions can maximize each function.

The CAC-OS or the CAC metal oxide includes conductive regions and insulating regions. The conductive regions have the above-described conducting function, and the insulating regions have the above-described insulating function. In some cases, the conductive regions and the insulating regions in the material are separated at the nanoparticle level. In some cases, the conductive regions and the insulating regions are unevenly distributed in the material. The conductive regions are observed to be coupled in a cloud-like manner with their boundaries blurred, in some cases.

Furthermore, in the CAC-OS or the CAC metal oxide, the conductive regions and the insulating regions each have a size of more than or equal to 0.5 nm and less than or equal to 10 nm, preferably more than or equal to 0.5 nm and less than or equal to 3 nm and are dispersed in the material, in some cases.

The CAC-OS or the CAC metal oxide includes components having different bandgaps. For example, the CAC-OS or the CAC metal oxide includes a component having a wide gap due to the insulating region and a component having a narrow gap due to the conductive region. In the case of such a composition, carriers mainly flow in the component having a narrow gap. The component having a narrow gap complements the component having a wide gap, and carriers also flow in the component having a wide gap in conjunction with the component having a narrow gap. Therefore, in the case where the above-described CAC-OS or the CAC metal oxide is used in a channel formation region of a transistor, high current drive capability in the on state of the transistor, that is, high on-state current and high field-effect mobility, can be obtained.

In other words, the CAC-OS or the CAC metal oxide can be called a matrix composite or a metal matrix composite.

<Structure of Metal Oxide>

An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis-aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion. Note that distortion refers to a portion where the direction of a lattice arrangement changes between a region with a uniform lattice arrangement and another region with a uniform lattice arrangement in a region where the nanocrystals are connected.

The shape of the nanocrystal is basically a hexagon but is not always a regular hexagon and is a non-regular hexagon in some cases. A pentagonal lattice arrangement, a heptagonal lattice arrangement, and the like are included in the distortion in some cases. Note that a clear grain boundary cannot be observed even in the vicinity of distortion in the CAAC-OS. That is, a lattice arrangement is distorted and thus formation of a grain boundary is inhibited. This is probably because the CAAC-OS can tolerate distortion owing to a low density of oxygen atom arrangement in an a-b plane direction, a change in interatomic bond distance by substitution of a metal element, and the like.

The CAAC-OS tends to have a layered crystal structure (also referred to as a stacked-layer structure) in which a layer containing indium and oxygen (hereinafter, In layer) and a layer containing the element M, zinc, and oxygen (hereinafter, (M, Zn) layer) are stacked. Note that indium and the element M can be replaced with each other, and when the element M of the (M, Zn) layer is replaced by indium, the layer can also be referred to as an (In, M, Zn) layer. When indium of the In layer is replaced by the element M, the layer can also be referred to as an (In, M) layer.

The CAAC-OS is an oxide semiconductor with high crystallinity. By contrast, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur because a clear grain boundary cannot be observed. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies). Thus, an oxide semiconductor including a CAAC-OS is physically stable. Therefore, the oxide semiconductor including a CAAC-OS is resistant to heat and has high reliability.

In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different nanocrystals in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method.

The a-like OS has a structure between those of the nc-OS and the amorphous oxide semiconductor. The a-like OS has a void or a low-density region. That is, the a-like OS has low crystallinity as compared with the nc-OS and the CAAC-OS.

An oxide semiconductor can have various structures that show various different properties. Two or more of the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, the a-like OS, the nc-OS, and the CAAC-OS may be included in an oxide semiconductor of one embodiment of the present invention.

<Transistor Containing Oxide Semiconductor>

Next, the case where the oxide semiconductor is used for a transistor is described.

When the oxide semiconductor is used in a transistor, the transistor can have high field-effect mobility. In addition, the transistor can have high reliability.

Moreover, the carrier density in the region 426 a of the oxide 406 b in the transistor is preferably low. In order to reduce the carrier density of the oxide semiconductor, the concentration of impurities in the oxide semiconductor is reduced so that the density of defect states can be reduced. In this specification and the like, a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state. The region 426 a of the oxide 406 b has, for example, a carrier density lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, and further preferably lower than 1×10¹⁰/cm³, and higher than or equal to 1×10⁻⁹/cm³.

A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has a low density of defect states and accordingly has a low density of trap states in some cases.

Charge trapped by the trap states in the oxide semiconductor takes a long time to be released and may behave like fixed charge. Thus, a transistor whose channel formation region is formed in the oxide semiconductor having a high density of trap states has unstable electrical characteristics in some cases.

In order to obtain stable electrical characteristics of the transistor, it is effective to reduce the concentration of impurities in the region 426 a of the oxide 406 b. In addition, in order to reduce the concentration of impurities in the region 426 a of the oxide 406 b, the concentration of impurities in a film that is adjacent to the region 426 a is preferably reduced. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, silicon, and the like are given.

<Impurity>

Here, the influence of impurities in the oxide semiconductor is described.

When silicon or carbon that is one of Group 14 elements is contained in the oxide, defect states are formed. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the region 426 a of the oxide 406 b is set to be lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

When the oxide semiconductor contains an alkali metal or an alkaline earth metal, defect states are formed and carriers are generated, in some cases. Thus, a transistor including an oxide semiconductor that contains an alkali metal or an alkaline earth metal is likely to be normally-on. Therefore, it is preferable to reduce the concentration of an alkali metal or an alkaline earth metal in the region 426 a of the oxide 406 b. Specifically, the concentration of alkali metal or alkaline earth metal in the region 426 a of the oxide 406 b, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³.

When the oxide semiconductor contains nitrogen, the oxide semiconductor easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor containing nitrogen in the region 426 a of the oxide 406 b tends to have normally-on characteristics. For this reason, nitrogen in the region 426 a of the oxide 406 b is preferably reduced as much as possible; for example, the concentration of nitrogen in the region 426 a of the oxide 406 b measured by SIMS is set to lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, and still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy, in some cases. Entry of hydrogen into the oxygen vacancy generates an electron serving as a carrier in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, the transistor including much hydrogen in the region 426 a of the oxide 406 b tends to have normally-on characteristics. For this reason, hydrogen in the region 426 a of the oxide 406 b is preferably reduced as much as possible. Specifically, the hydrogen concentration of the oxide semiconductor measured by SIMS is lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, and still further preferably lower than 1×10¹⁸ atoms/cm³.

By reducing impurities in the region 426 a of the oxide 406 b to an enough level, the transistor can have stable electrical characteristics.

Modification Example 1

The structure of the semiconductor device in this embodiment is not limited to the structure in FIGS. 1A to 1C. Modification examples of the transistor described in this embodiment are described below with reference to FIGS. 2A to 2C and FIGS. 7A and 7B.

FIG. 2A is a top view of a semiconductor device including a transistor 1000 a. FIG. 2B is a cross-sectional view taken along dashed-dotted line A1-A2 in FIG. 2A, illustrating a cross-sectional view of the transistor 1000 a in the channel length direction. FIG. 2C is a cross-sectional view of a portion indicated by a dashed-dotted line A3-A4 in FIG. 2A, illustrating a cross-sectional view of the transistor 1000 a in a channel width direction. Note that for simplification of the drawing, some components in the top view in FIG. 2A are not illustrated.

The transistor 1000 a differs from the transistor 1000 in that the conductors 450 a, 451 a, and 452 a and the conductors 450 b, 451 b, and 452 b are shifted in opposite directions to each other along the channel width direction. The description of the transistor 1000 can be referred to for the other components.

In the structure of the transistor 1000 illustrated in FIGS. 1A to 1C, with the miniaturization of the transistor, end portions of the conductors 452 a and 452 b that face each other may become further close to each other. The distance between the end portions facing each other in such an arrangement may be smaller than the resolution limit (in this case, the resolution limit of the distance). In the case where the distance is smaller than the resolution limit, the conductors 452 a and 452 b may be short-circuited; thus, the position cannot be used.

Therefore, by shifting the conductors 450 a, 451 a, and 452 a and the conductors 450 b, 451 b, and 452 b in the opposite directions to each other along the channel width direction as illustrated in FIG. 2A, a miniaturized arrangement in which the distance between the end portions of the conductors 452 a and 452 b that face each other is not smaller than the resolution limit of a lithography method is possible.

Although the conductors 450 a, 451 a, and 452 a and the conductors 450 b, 451 b, and 452 b are shifted in opposite directions to each other along the channel width direction as an example in this embodiment, only the conductors 450 a, 451 a, and 452 a may be shifted. Alternatively, only the conductors 450 b, 451 b, and 452 b may be shifted.

FIG. 7A is a cross-sectional view taken along dashed-dotted line A5-A6 in FIG. 2A, and FIG. 7B is a cross-sectional view taken along dashed-dotted line A7-A8 in FIG. 2A.

As illustrated in FIG. 2B and FIG. 7A, the conductor 450 b is in contact with at least the top surface of the oxide 406 and is preferably in contact with a rounded corner of the oxide 406. In particular, as illustrated in FIG. 7A, the conductor 450 b is preferably in contact with part of a rounded corner on the A6 side in the channel width direction of the oxide 406. Furthermore, as illustrated in FIG. 2B, the conductor 450 b may be in contact with part of a rounded corner on the A2 side in the channel length direction of the oxide 406.

With such a structure in which the conductor 450 b is in contact with the part of the rounded corner of the oxide 406 as well as the top surface of the oxide 406 in the arrangement where the conductors 450 b, 451 b, and 452 b are shifted to the A6 side in the channel width direction as illustrated in FIG. 7A, a sufficient contact area between the conductor 450 b and the oxide 406 can be secured.

Similarly, as illustrated in FIG. 2B and FIG. 7B, the conductor 450 a is in contact with at least the top surface of the oxide 406 and is preferably in contact with a rounded corner of the oxide 406. In particular, as illustrated in FIG. 7B, the conductor 450 a is preferably in contact with part of a rounded corner on the A7 side in the channel width direction of the oxide 406. Alternatively, as illustrated in FIG. 2B, the conductor 450 a may be in contact with part of a rounded corner on the A1 side in the channel length direction of the oxide 406.

With such a structure in which the conductor 450 a is in contact with the part of the rounded corner of the oxide 406 as well as the top surface of the oxide 406 even when the conductors 450 a, 451 a, and 452 a are shifted to the A7 side in the channel width direction as illustrated in FIG. 7B, a sufficient contact area between the conductor 450 a and the oxide 406 can be secured.

Accordingly, miniaturization of the source electrode and the drain electrode of the transistor can be achieved and, in addition, the on-state current can be increased. The description of the transistor 1000 can be referred to for the other components and effects.

Modification Example 2

Hereinafter, a modification example of the transistor described in this embodiment is described with reference to FIGS. 3A to 3C.

FIG. 3A is a top view of a semiconductor device including a transistor 1000 b. FIG. 3B is a cross-sectional view taken along dashed-dotted line A1-A2 in FIG. 3A, illustrating a cross-sectional view of the transistor 1000 b in the channel length direction. FIG. 3C is a cross-sectional view of a portion indicated by a dashed-dotted line A3-A4 in FIG. 3A, illustrating a cross-sectional view of the transistor 1000 b in a channel width direction. For simplification of the drawing, some components in the top view in FIG. 3A are not illustrated.

The transistor 1000 b differs from the transistor 1000 illustrated in FIGS. 1A to 1C in that the transistor has a plurality of channel formation regions for one gate electrode. Owing to the plurality of channel formation regions, the transistor 1000 b can have a large on-state current. Furthermore, each channel formation region is surrounded by the gate electrode, in other words, an s-channel structure is employed; thus, a large on-state current can be obtained in each channel formation region. Although the transistor in FIGS. 3A to 3C includes three channel formation regions, the number of the channel formation regions is not limited to three. The description of the transistor 1000 illustrated in FIGS. 1A to 1C can be referred to for the other components.

<Method for Manufacturing Semiconductor Device>

Next, a method for manufacturing a semiconductor device including the transistor 1000 of the present invention is described with reference to FIGS. 8A to 8C to FIG. 20. In FIGS. 8A to 8C to FIGS. 19A to 19C, Figure A is a top view. Note that in the top views, some components are not illustrated for simplification of the drawings. Figure B is a cross-sectional view taken along the dashed-dotted line A1-A2 in Figure A. Figure C is a cross-sectional view taken along the dashed-dotted line A3-A4 in Figure A.

First, the substrate (not shown) is prepared, and the insulator 432 is deposited over the substrate. The insulator 432 can be deposited by a sputtering method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, an ALD method, or the like.

Note that CVD methods can be classified into a plasma enhanced CVD (PECVD) method using plasma, a thermal CVD (TCVD) method using heat, a photo CVD method using light, and the like. Moreover, the CVD methods can be classified into a metal CVD (MCVD) method and a metal organic CVD (MOCVD) method depending on a source gas.

The use of a PECVD method can provide a high-quality film at a relatively low temperature. A thermal CVD method does not use plasma and thus causes less plasma damage to an object. A wiring, an electrode, an element (e.g., a transistor or a capacitor), or the like included in a semiconductor device might be charged up by receiving charges from plasma, for example. In that case, accumulated charges might break the wiring, electrode, element, or the like included in the semiconductor device. By contrast, when a thermal CVD method not using plasma is employed, such plasma damage is not caused and the yield of semiconductor devices can be increased. A thermal CVD method does not cause plasma damage during deposition, so that a film with few defects can be obtained.

An ALD method also causes less plasma damage to an object. Since an ALD method does not cause plasma damage during deposition, a film with few defects can be obtained.

Unlike in a deposition method in which particles ejected from a target or the like are deposited, in a CVD method and an ALD method, a film is formed by reaction at a surface of an object. Thus, a CVD method and an ALD method can provide favorable step coverage almost regardless of the shape of an object. In particular, an ALD method can provide excellent step coverage and excellent thickness uniformity and thus can be favorably used for covering a surface of an opening with a high aspect ratio, for example. On the other hand, an ALD method has a relatively low deposition rate; thus, it is sometimes preferable to combine an ALD method with another deposition method with a high deposition rate such as a CVD method.

When a CVD method or an ALD method is used, the composition of a film to be formed can be controlled with the flow rate ratio of a source gas. For example, by a CVD method or an ALD method, a film with a certain composition can be formed depending on the flow rate ratio of a source gas. Moreover, by changing the flow rate ratio of a source gas during deposition by a CVD method or an ALD method, a film whose composition is continuously changed can be formed. In the case where a film is formed while changing the flow rate ratio of a source gas, as compared to the case where a film is formed using a plurality of deposition chambers, time taken for the deposition can be reduced because time taken for transfer and pressure adjustment is omitted. Thus, semiconductor devices can be manufactured with improved productivity in some cases.

In this embodiment, as the insulator 432, aluminum oxide is deposited by a sputtering method. The insulator 432 may have a multilayer structure. For example, aluminum oxide may be deposited by a sputtering method and another aluminum oxide may be deposited by an ALD method over the aluminum oxide. Alternatively, aluminum oxide may be deposited by an ALD method and another aluminum oxide may be deposited by a sputtering method over the aluminum oxide.

Next, the insulator 430 is deposited over the insulator 432. The insulator 430 can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, as the insulator 430, silicon oxide is deposited by a CVD method.

Then, a depression is formed in the insulator 430 so as to reach the insulator 432. Examples of the depression include a hole and an opening. The depression may be formed by wet etching; however, dry etching is preferred for microfabrication. The insulator 432 is preferably an insulator that serves as an etching stopper film when forming the depression by etching the insulator 430. For example, in the case where a silicon oxide film is used as the insulator 430 in which the depression is to be formed, a silicon nitride film, an aluminum oxide film, or a hafnium oxide film is used as the insulator 432.

After the formation of the depression, a conductor to be the conductor 440 a is deposited. The conductor to be the conductor 440 a desirably contains a conductor that has a function of inhibiting the penetration of oxygen. For example, tantalum nitride, tungsten nitride, or titanium nitride can be used. Alternatively, a stacked-layer film formed using the conductor and tantalum, tungsten, titanium, molybdenum, aluminum, copper, or a molybdenum-tungsten alloy can be used. The conductor to be the conductor 440 a can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

In this embodiment, a film of tantalum nitride or a film including tantalum nitride and titanium nitride stacked thereover is deposited by a sputtering method as the conductor to be the conductor 440 a. In the case where such a metal nitride is used as the conductor 440 a, even when a metal that is easy to diffuse (e.g., copper) is used for the conductor 440 b described later, the metal can be prevented from diffusing outward through the conductor 440 a.

Then, a conductor to be the conductor 440 b is deposited over the conductor to be the conductor 440 a. The conductor to be the conductor 440 b can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, a low-resistance conductive material such as copper is deposited as the conductor to be the conductor 440 b.

Then, CMP treatment is performed so that the conductor to be the conductor 440 a and the conductor to be the conductor 440 b that are above the insulator 430 are removed. Consequently, the conductor to be the conductor 440 a and the conductor to be the conductor 440 b are left only in the depression, whereby the conductor 440 that includes the conductor 440 a and the conductor 440 b with flat top surfaces can be formed (see FIGS. 8A to 8C).

Then, the insulator 401 is deposited over the conductor 440 and the insulator 430. The insulator 401 can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, as the insulator 401, silicon nitride is deposited by a CVD method. As described here, an insulator that is unlikely to allow the penetration of copper, such as silicon nitride, is used as the insulator 401; accordingly, even when a metal that is easy to diffuse (e.g., copper) is used for the conductor 440 b, the metal can be prevented from diffusing to layers over the insulator 401.

Next, the insulator 301 is deposited over the insulator 401. The insulator 301 can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, as the insulator 301, silicon oxide is deposited by a CVD method.

Next, a depression that reaches the conductor 440 is formed in the insulators 401 and 301. Examples of the depression include a hole and an opening. The depression may be formed by wet etching; however, dry etching is preferred for microfabrication.

After the formation of the depression, a conductor to be the conductor 310 a is deposited. The conductor to be the conductor 310 a desirably contains a conductor that has a function of inhibiting the penetration of oxygen. For example, tantalum nitride, tungsten nitride, or titanium nitride can be used. Alternatively, a stacked-layer film formed using the conductor and tantalum, tungsten, titanium, molybdenum, aluminum, copper, or a molybdenum-tungsten alloy can be used. The conductor to be the conductor 310 a can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

In this embodiment, tantalum nitride is deposited by a sputtering method for the conductor to be the conductor 310 a.

Then, a conductor to be the conductor 310 b is deposited over the conductor to be the conductor 310 a. The conductor to be the conductor 310 b can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

In this embodiment, for the conductor to be the conductor 310 b, titanium nitride is deposited by a CVD method and tungsten is deposited by a CVD method over the titanium nitride.

Next, CMP treatment is performed to remove the conductor to be the conductor 310 a and the conductor to be the conductor 310 b that are located over the insulator 301. Consequently, the conductor to be the conductor 310 a and the conductor to be the conductor 310 b are left only in the depression, whereby the conductor 310 that includes the conductor 310 a and the conductor 310 b with flat top surfaces can be formed (see FIGS. 8A to 8C).

Then, the insulator 302 is deposited over the insulator 301 and the conductor 310. The insulator 302 can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Subsequently, the insulator 303 is deposited over the insulator 302. The insulator 303 can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

After that, the insulator 402 is deposited over the insulator 303. The insulator 402 can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Next, first heat treatment is preferably performed. The first heat treatment can be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C., and further preferably higher than or equal to 320° C. and lower than or equal to 450° C. The first heat treatment is performed in a nitrogen atmosphere, an inert gas atmosphere, or an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more. The first heat treatment may be performed under a reduced pressure. Alternatively, the first heat treatment may be performed in such a manner that heat treatment is performed in a nitrogen atmosphere or an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more in order to compensate for released oxygen. By the first heat treatment, impurities such as hydrogen or water contained in the insulator 402 can be removed, for example. In the first heat treatment, plasma treatment using oxygen may be performed under a reduced pressure. The plasma treatment using oxygen is preferably performed using an apparatus including a power source for generating high-density plasma using microwaves, for example. Alternatively, a power source for applying a radio frequency (RF) to a substrate side may be provided. The use of high-density plasma enables high-density oxygen radicals to be produced, and application of the RF to the substrate side allows oxygen radicals generated by the high-density plasma to be efficiently introduced into the insulator 402. Alternatively, after plasma treatment using an inert gas is performed with the apparatus, plasma treatment using oxygen may be performed in order to compensate for released oxygen. Note that the first heat treatment is not necessary in some cases.

Alternatively, heat treatment can be performed after the formation of the insulator 302, after the formation of the insulator 303, and after the formation of the insulator 402. Although each heat treatment can be performed under the conditions for the first heat treatment, the heat treatment after the formation of the insulator 302 is preferably performed in an atmosphere containing nitrogen.

In this embodiment, the first heat treatment is performed in such a manner that treatment at 400° C. in a nitrogen atmosphere is performed for one hour after the insulator 402 is formed and then treatment at 400° C. in an oxygen atmosphere is successively performed for one hour.

Next, an oxide film 406 aA and an oxide film 406 bA are deposited in this order over the insulator 402 (see FIGS. 8A to 8C). Note that it is preferable to deposit the oxide film 406 aA and the oxide film 406 bA successively without exposure to the air. In that case, an impurity or moisture in the air can be prevented from being attached onto the oxide film 406 aA, and the interface between the oxide film 406 aA and the oxide film 406 bA and the vicinity thereof can be kept clean.

The oxide films 406 aA and 406 bA can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

In the case where the oxide films 406 aA and 406 bA are deposited by a sputtering method, for example, oxygen or a mixed gas of oxygen and a rare gas is used as a sputtering gas. When the proportion of oxygen in the sputtering gas is increased, the amount of excess oxygen in the oxide films to be formed can be increased. In the case where the oxide films 406 aA and 406 bA are deposited by a sputtering method, the above-described In-M-Zn oxide target can be used.

In particular, part of oxygen contained in the sputtering gas is supplied to the insulator 402 in some cases, at the formation of the oxide film 406 aA.

Note that the proportion of oxygen contained in the sputtering gas for the oxide film 406 aA is 70% or higher, preferably 80% or higher, and further preferably 100%.

The oxide film 406 bA is formed by a sputtering method. At this time, when the proportion of oxygen in the sputtering gas is higher than or equal to 1% and lower than or equal to 30%, preferably higher than or equal to 5% and lower than or equal to 20%, an oxygen-deficient oxide semiconductor is formed. A transistor including an oxygen-deficient oxide semiconductor can have relatively high field-effect mobility.

In the case where an oxygen-deficient oxide semiconductor is used for the oxide film 406 bA, an oxide film containing excess oxygen is preferably used as the oxide film 406 aA. Oxygen doping treatment may be performed after the formation of the oxide film 406 bA.

In this embodiment, the oxide film 406 aA is deposited by a sputtering method using a target containing In, Ga, and Zn at an atomic ratio of 1:3:4, and the oxide film 406 bA is deposited by a sputtering method using a target containing In, Ga, and Zn at an atomic ratio of 4:2:4.1.

After that, second heat treatment may be performed. For the second heat treatment, the conditions for the first heat treatment can be used. By the second heat treatment, impurities such as hydrogen or water contained in the oxide films 406 aA and 406 bA can be removed, for example. In this embodiment, the second heat treatment is performed in such a manner that treatment at 400° C. in a nitrogen atmosphere is performed for one hour and then treatment at 400° C. in an oxygen atmosphere is successively performed for one hour.

Then, the oxide films 406 aA and 406 bA are processed into island shapes to form the oxides 406 a and 406 b (see FIGS. 9A to 9C). Here, the oxides 406 a and 406 b are formed so that at least parts thereof overlap with the conductor 310. The oxide films 406 aA and 406 bA can be processed by a lithography method. After the processing, upper corners of the oxide 406 b are preferably rounded in a cross section. The curvature radius of each of the rounded corners is preferably greater than or equal to one-fourth of the total thickness of the oxides 406 a and 406 b and less than or equal to the total thickness of the oxides 406 a and 406 b. Owing to such rounded corners, the coverage with films formed later in the manufacturing process can be improved. The processing can be performed by a dry etching method or a wet etching method. A dry etching method is suitably used for microfabrication and the above-described processing into a curved shape.

In the lithography method, first, a resist is exposed to light through a mask. Next, a region exposed to light is removed or left using a developing solution, so that a resist mask is formed. Then, etching is conducted with the resist mask. As a result, a conductor, a semiconductor, an insulator, or the like can be processed into a desired shape. The resist mask is formed by, for example, exposure of the resist to light such as KrF excimer laser light, ArF excimer laser light, or extreme ultraviolet (EUV) light. A liquid immersion technique may be employed in which a portion between a substrate and a projection lens is filled with liquid (e.g., water) to perform light exposure. An electron beam or an ion beam may be used instead of the above-mentioned light. Note that a mask is not necessary in the case of using an electron beam or an ion beam. To remove the resist mask, dry etching treatment such as ashing or wet etching treatment can be used. Alternatively, wet etching treatment can be performed after dry etching treatment. Further alternatively, dry etching treatment can be performed after wet etching treatment.

A hard mask formed of an insulator or a conductor may be used instead of the resist mask. In the case where a hard mask is used, a hard mask with a desired shape can be formed in the following manner: an insulating film or a conductive film that is the material of the hard mask is formed over the oxide film 406 bA, a resist mask is formed thereover, and then the material of the hard mask is etched. The etching of the oxide films 406 aA and 406 bA may be performed after or without removal of the resist mask. In the latter case, the resist mask may be removed during the etching. The hard mask may be removed by etching after the etching of the oxide films 406 aA and 406 bA. The hard mask does not need to be removed in the case where the material of the hard mask does not affect the following process or can be utilized in the following process.

As a dry etching apparatus, a capacitively coupled plasma (CCP) etching apparatus including parallel plate electrodes can be used. The capacitively coupled plasma etching apparatus including parallel plate electrodes may have a structure in which high-frequency power is applied to one of the parallel plate electrodes. Alternatively, different high-frequency powers are applied to one of the parallel plate electrodes. Further alternatively, high-frequency powers with the same frequency are applied to the parallel plate electrodes. Still further alternatively, high-frequency powers with different frequencies are applied to the parallel plate electrodes. Alternatively, a dry etching apparatus including a high-density plasma source can be used. As the dry etching apparatus including a high-density plasma source, an inductively coupled plasma (ICP) etching apparatus can be used, for example.

In some cases, treatment such as dry etching performed in the above process causes the attachment or diffusion of impurities due to an etching gas or the like to a surface or an inside of the oxide 406 a, the oxide 406 b, or the like. Examples of the impurities include fluorine and chlorine.

To remove the impurities or the like, cleaning is performed. As the cleaning, any of wet cleaning using a cleaning solution or the like, plasma treatment using plasma, cleaning by heat treatment, and the like may be performed in appropriate combination.

The wet cleaning may be performed using an aqueous solution in which oxalic acid, phosphoric acid, hydrofluoric acid, or the like is diluted with carbonated water or pure water. Alternatively, ultrasonic cleaning using pure water or carbonated water may be performed. In this embodiment, ultrasonic cleaning using pure water or carbonated water is performed.

Next, third heat treatment may be performed. For the third heat treatment, the conditions for the first heat treatment can be used. Note that the third heat treatment is not necessary in some cases. In this embodiment, the third heat treatment is not performed.

Then, an insulating film 412A, a conductive film 404 aA, a conductive film 404 bA, and an insulating film 419A are deposited in this order over the insulator 402 and the oxides 406 a and 406 b (see FIGS. 10A to 10C).

The insulating film 412A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Here, fourth heat treatment can be performed. For this heat treatment, the conditions for the first heat treatment can be used. The fourth heat treatment can reduce the moisture concentration and the hydrogen concentration in the insulating film 412A. Note that the fourth heat treatment is not necessary in some cases.

The conductive film 404 aA can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. By depositing the above-described conductive oxide that can be used for the conductor 404 a by a sputtering method in an oxygen-containing atmosphere to form the conductive film 404 aA, oxygen can be add to the insulator 412, which makes it possible to supply oxygen to the oxide 406 b.

The conductive film 404 bA can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. By forming the conductive film 404 bA by a sputtering method, the conductive film 404 aA can have reduced electric resistance and become a conductor. Such a conductor can be called an oxide conductor (OC) electrode. Another conductor may be deposited by a sputtering method or the like over the conductor over the OC electrode.

The insulating film 419A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like; and in particular, an ALD method is preferably used. When the insulating film 419A is deposited by an ALD method, the film thickness can be approximately 1 nm to 20 nm inclusive, preferably approximately 5 nm to 10 nm inclusive. The thickness of the insulating film 419A is preferably larger than that of the insulating film 418A to be the insulator 418. In that case, the insulator 419 is likely to be left over the conductor 404 in a later formation step of the insulator 418.

Here, fifth heat treatment can be performed. For the fifth heat treatment, the conditions for the first heat treatment can be used. Note that the fifth heat treatment is not necessary in some cases.

Next, the insulating film 412A, the conductive film 404 aA, the conductive film 404 bA, and the insulating film 419A are etched to form the insulator 412, the conductor 404 a, the conductor 404 b, and the insulator 419 (see FIGS. 11A to 11C). The insulator 412, the conductor 404 a, the conductor 404 b, and the insulator 419 are formed so that at least parts thereof overlap with the conductor 310 and the oxide 406. The processing of the insulating film 412A, the conductive film 404 aA, the conductive film 404 bA, and the insulating film 419A are performed by using a lithography method.

Here, a cross section of the insulator 412, the conductor 404 a, the conductor 404 b, and the insulator 419 is preferably tapered as little as possible. The angle formed between the bottom surface of the oxide 406 and each of the side surfaces of the insulator 412, the conductor 404 a, the conductor 404 b, and the insulator 419 is preferably greater than or equal to 80° and less than or equal to 100°. In that case, the insulator 418 is likely to be left in the later formation step of the insulator 418.

Note that an upper portion of the oxide 406 b in a region not overlapping with the insulator 412 may be etched by the above etching. In that case, the oxide 406 b is thicker in the region overlapping with the insulator 412 than in the region not overlapping with the insulator 412.

Next, the insulating film 418A is deposited by an ALD method so as to cover the insulator 402, the oxide 406, the insulator 412, the conductor 404, and the insulator 419 (see FIGS. 12A to 12C). When the insulating film 418A is deposited by an ALD method, the thickness of the insulating film 418A can be approximately 1 nm to 20 nm inclusive, preferably 1 nm to 3 nm inclusive (e.g., approximately 1 nm). Furthermore, even when the aspect ratio of a structure body formed of the insulator 412, the conductor 404, and the insulator 419 is extremely high, the insulating film 418A deposited by an ALD method can have few pinholes and uniform thickness on the top surface and a side surface of the structure body. In this embodiment, aluminum oxide is deposited by an ALD method as the insulating film 418A.

Then, anisotropic etching is performed on the insulating film 418A to form the insulator 418 on side surfaces of the insulator 412, the conductor 404, and the insulator 419 (see FIGS. 13A to 13C). Dry etching is preferably performed as the anisotropic etching. In this manner, the insulating film 418A in a region on a plane substantially parallel to the substrate surface can be removed, so that the insulator 418 can be formed in a self-aligned manner.

Here, the thickness of the insulator 419 is made larger than that of the insulating film 418A, so that part of the insulator 419 and part of the insulating film 418A can be left even when upper portions of the insulator 419 and the insulating film 418A are removed. A portion of the insulating film 418A that is deposited in contact with the side surface of the oxide 406 is left on the end portion of the oxide 406 to be the insulator 410.

The insulator 410 on the side surface of the oxide 406 can reduce impurities such as water or hydrogen that enter the oxide 406 and can prevent outward diffusion of oxygen from the oxide 406, in some cases.

Next, the insulating film 409A is deposited so as to cover the insulator 402, the oxide 406, the insulator 418, and the insulator 419 (see FIGS. 14A to 14C). The insulating film 409A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

The insulating film 409A is preferably deposited in an atmosphere containing at least one of nitrogen and hydrogen. In that case, oxygen vacancies are formed mainly in the region of the oxide 406 b not overlapping with the insulator 412 and the oxygen vacancies and impurity elements such as nitrogen or hydrogen are bonded to each other, leading to an increase in carrier density. In this manner, the regions 426 b and 426 c with reduced resistance can be formed. For the insulating film 409A, for example, silicon nitride or silicon nitride oxide can be deposited by a CVD method. In this embodiment, silicon nitride oxide is used for the insulating film 409A.

As described above, in the method for manufacturing a semiconductor device described in this embodiment, a source region and a drain region can be formed in a self-aligned manner owing to the formation of the insulating film 409A, even in a minute transistor whose channel length is approximately 10 nm to 30 nm. Thus, minute or highly integrated semiconductor devices can be manufactured with high yield.

Here, the top and side surfaces of the conductor 404 and the insulator 412 are covered with the insulators 419 and 418, whereby impurity elements such as nitrogen or hydrogen can be prevented from entering the conductor 404 and the insulator 412. Thus, impurity elements such as nitrogen or hydrogen can be prevented from entering the region 426 a serving as a channel formation region through the conductor 404 and the insulator 412, so that a transistor with good electrical characteristics can be provided.

Plasma treatment may be performed before the insulating film 409A is deposited. The plasma treatment is performed in an atmosphere containing an element that forms the oxygen vacancies or an element bonded to the oxygen vacancies, for example.

The regions 426 b and 426 c may be formed in the oxide 406 only by plasma treatment.

Then, anisotropic etching is performed on the insulating film 409A to form the insulator 409 on side surfaces of the insulator 418 (see FIGS. 15A to 15C). Dry etching is preferably performed as the anisotropic etching. In this manner, the insulating film 409A in a region on a plane substantially parallel to the substrate surface can be removed, so that the insulator 409 can be formed in a self-aligned manner.

Here, the thickness of the insulator 419 is made larger than that of the insulating film 418A, so that part of the insulator 419 and part of the insulating film 418A can be left even when upper portions of the insulator 419 and the insulating film 418A are removed. The insulating film 409A deposited in contact with part of the side surface of the oxide 406 and the side surface of the insulator 410 is left on the side surface of the insulator 410 to be the insulator 420.

The insulator 420 on part of the side surface of the oxide 406 and the side surface of the insulator 410 can prevent outward diffusion of oxygen from the oxide 406, in some cases.

Next, an insulating film 415A is deposited over the insulator 409 (see FIGS. 16A to 16C). The insulating film 415A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. Alternatively, the insulating film 415A can be formed by a spin coating method, a dipping method, a droplet discharging method (such as an ink-jet method), a printing method (such as screen printing or offset printing), a doctor knife method, a roll coater method, a curtain coater method, or the like. In this embodiment, silicon oxynitride is used for the insulating film 415A.

Next, the insulating film 415A is partly removed to form the insulator 415 (see FIGS. 17A to 17C). The insulator 415 is preferably formed to have a flat top surface. The top surface of the insulator 415 may be flat immediately after the deposition (i.e., when deposited as the insulating film 415A), for example. Alternatively, the insulator 415 may have flatness in the following manner, for example: an insulator and the like are removed from the top surface after the deposition so that the top surface of the insulator 415 becomes parallel to a reference surface such as the rear surface of the substrate. Such treatment is referred to as planarization treatment. As the planarization treatment, for example, CMP treatment, dry etching treatment, or the like can be performed. In this embodiment, CMP treatment is performed as the planarization treatment. The arrows in FIGS. 17B and 17C indicate the direction in which the CMP treatment proceeds. Note that the top surface of the insulator 415 need not necessarily be flat.

Next, an opening reaching the region 426 b of the oxide 406 and an opening reaching the region 426 c of the oxide 406 are formed in the insulator 415 (see FIGS. 18A to 18C). The opening may be formed by a lithography method. The openings are formed so that the conductors 450 a and 450 b are formed in contact with the side surface of the insulator 409, the side surface of the oxide 406 b, and the side surface of the insulator 420. The openings are preferably formed under a condition where the insulator 409 is hardly etched, in other words, the etching rate of the insulator 415 is preferably higher than that of the insulator 409. When the etching rate of the insulator 409 is set to 1, the etching rate of the insulator 415 is preferably set to 5 or more, further preferably 10 or more. With such an opening condition, the openings can be positioned near the region 426 a; thus, a miniaturized transistor can be manufactured. Tolerance of misalignment between the conductor 404 and the openings and between the oxide 406 and the openings can be extended in a lithography process; thus, the yield should be improved.

Next, a conductor to be the conductor 450 a and the conductor 450 b is deposited. The conductor to be the conductor 450 a and the conductor 450 b preferably contains a conductor having a function of inhibiting the penetration of impurities such as water or hydrogen. For example, tantalum nitride, titanium nitride, or the like can be used. The conductor to be the conductor 450 a and the conductor 450 b can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Then, a conductor to be the conductor 451 a and the conductor 451 b is deposited over the conductor to be the conductor 450 a and the conductor 450 b. The conductor to be the conductor 451 a and the conductor 451 b can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like.

Then, the conductor to be the conductor 450 a and the conductor 450 b and the conductor to be the conductor 451 a and the conductor 451 b in portions above the insulator 415 are removed by CMP treatment. As a result, the conductors are left only in the above-described openings, so that the conductors 450 a and 451 a and the conductors 450 b and 451 b with flat top surfaces can be formed.

Next, a conductor is deposited and processed by using a photolithography method, whereby the conductor 452 a and the conductor 452 b are formed (see FIGS. 19A to 19C). The conductor to be the conductor 452 a and the conductor 452 b can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. The conductor 452 a and the conductor 452 b may be embedded in the insulator, in a similar manner to the conductor 440 or the like.

Through the above-described process, a semiconductor device including the transistor 1000 can be manufactured (see FIGS. 1A to 1C).

In the above-described method for manufacturing a semiconductor device, the insulator 409 in contact with the oxide 406 is formed to form the regions 426 b and 426 c; however, the method for manufacturing a semiconductor device of this embodiment is not limited thereto. For example, a dopant may be added as illustrated in FIG. 20 to form the regions 426 b and 426 c.

The step illustrated in FIG. 20 is performed after the formation of the insulator 418 illustrated in FIGS. 13A to 13C. As illustrated in FIG. 20, a dopant 422 is added to the oxide 406 using the insulator 412, the conductor 404, and the insulator 418 as masks.

For the addition of the dopant 422, an ion implantation method by which an ionized source gas is subjected to mass separation and then added, an ion doping method by which an ionized source gas is added without mass separation, a plasma immersion ion implantation method, or the like can be used. In the case of performing mass separation, ion species to be added and its concentration can be controlled properly. On the other hand, in the case of not performing mass separation, ions at a high concentration can be added in a short time. Alternatively, an ion doping method in which atomic or molecular clusters are generated and ionized may be employed. Instead of the term “dopant”, the term “ion”, “donor”, “acceptor”, “impurity”, “element”, or the like may be used.

As the dopant 422, the element that forms an oxygen vacancy, the element bonded to an oxygen vacancy, or the like is used. Typical examples of the element are hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, and a rare gas element. Typical examples of the rare gas element are helium, neon, argon, krypton, and xenon.

As described above, when the indium content in the oxide 406 is increased, the carrier density is increased and the resistance can be decreased. Thus, as the dopant 422, a metal element such as indium that improves the carrier density of the oxide 406 can be used. Here, it is preferable that the dopant 422 be added so that the peak of the concentration of indium is in the oxide 406 a.

By adding indium in such a manner, it is preferable in the oxide 406 a that the atomic ratio of indium to the element M in the regions 426 b and 426 c be substantially the same as that in the oxide 406 b. In other words, in the oxide 406 a, the atomic ratio of indium to the element M in the regions 426 b and 426 c is preferably greater than that in the region 426 a.

With indium added in the above manner, even when the thickness of the oxide 406 b is small and electric resistance of the oxide 406 b is high in the manufacturing process of the transistor 1000, the region 426 b and the region 426 c in the oxide 406 can function as a source region and a drain region owing to the sufficiently reduced resistance of the oxide 406 a in the region 426 b and the region 426 c.

An insulating material having a function of inhibiting the penetration of oxygen and impurities such as water or hydrogen, as with the insulator 409 or the like, is preferably formed after the regions 426 b and 426 c are formed in the oxide 406 in the above-described manner. When such an insulator is provided over the regions 426 b and 426 c, oxygen and impurities such as water or hydrogen can be prevented from entering the regions 426 b and 426 c, leading to the prevention of a change in carrier density.

As described above, according to one embodiment of the present invention, a semiconductor device that can be miniaturized or highly integrated, a semiconductor device having good electrical characteristics, a semiconductor device with low off-state current, a transistor with high on-state current, a highly reliable semiconductor device, a semiconductor device with low power consumption, or a semiconductor device that can be manufactured with high productivity can be provided.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device is described with reference to FIGS. 22 to 25.

[Memory Device 1]

Semiconductor devices illustrated in FIG. 22 to FIG. 25 each include a transistor 300, a transistor 200, and a capacitor 100.

The transistor 200 is a transistor in which a channel is formed in a semiconductor layer containing an oxide semiconductor, and can be the transistor described in the above embodiment. Since the transistor described in the above embodiment can be formed with high yield even when it is miniaturized, the transistor 200 can be miniaturized. The use of such a transistor in a memory device allows miniaturization or high integration of the memory device. Since the off-state current of the transistor described in the above embodiment is low, a memory device including the transistor can retain stored data for a long time. In other words, such a memory device does not require refresh operation or has an extremely low frequency of the refresh operation, which leads to a sufficient reduction in power consumption of the memory device.

In each of FIG. 22 to FIG. 25, a wiring 3001 is electrically connected to a source of the transistor 300. A wiring 3002 is electrically connected to a drain of the transistor 300. A wiring 3003 is electrically connected to one of a source and a drain of the transistor 200. A wiring 3004 is electrically connected to a first gate of the transistor 200. A wiring 3006 is electrically connected to a second gate of the transistor 200. A gate of the transistor 300 and the other of the source and the drain of the transistor 200 are electrically connected to one electrode of the capacitor 100. A wiring 3005 is electrically connected to the other electrode of the capacitor 100.

The semiconductor devices illustrated in FIG. 22 to FIG. 25 each have a feature that the potential of the gate of the transistor 300 can be retained and thus enable writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of the wiring 3004 is set to a potential at which the transistor 200 is turned on, so that the transistor 200 is turned on. Accordingly, the potential of the wiring 3003 is supplied to a node FG where the gate of the transistor 300 and the one electrode of the capacitor 100 are electrically connected to each other. That is, a predetermined charge is supplied to the gate of the transistor 300 (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the wiring 3004 is set to a potential at which the transistor 200 is turned off, so that the transistor 200 is turned off. Thus, the charge is retained in the node FG (retaining).

In the case where the off-state current of the transistor 200 is low, the charge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (reading potential) is supplied to the wiring 3005 while a predetermined potential (constant potential) is supplied to the wiring 3001, whereby the potential of the wiring 3002 varies depending on the amount of charge retained in the node FG. This is because in the case of using an n-channel transistor as the transistor 300, an apparent threshold voltage V_(th) _(_) _(H) at the time when a high-level charge is given to the gate of the transistor 300 is lower than an apparent threshold voltage V_(th) _(_) _(L) at the time when a low-level charge is given to the gate of the transistor 300. Here, an apparent threshold voltage refers to the potential of the wiring 3005 that is needed to turn on the transistor 300. Thus, the potential of the wiring 3005 is set to a potential V₀ that is between V_(th) _(_) _(H) and V_(th) _(_) _(L), whereby the charge supplied to the node FG can be determined. For example, in the case where a high-level charge is supplied to the node FG in writing and the potential of the wiring 3005 is V₀ (>V_(th) _(_) _(H)), the transistor 300 is turned on. Meanwhile, in the case where a low-level charge is supplied to the node FG in writing, even when the potential of the wiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 300 remains off. Thus, the data retained in the node FG can be read by determining the potential of the wiring 3002.

<Structure of Semiconductor Device 1>

The semiconductor device of one embodiment of the present invention includes the transistor 300, the transistor 200, and the capacitor 100 as illustrated in FIG. 22. The transistor 200 is provided above the transistor 300, and the capacitor 100 is provided above the transistor 300 and the transistor 200.

The transistor 300 is provided over a substrate 311 and includes a conductor 316, an insulator 315, a semiconductor region 313 that is a part of the substrate 311, and a low-resistance region 314 a and a low-resistance region 314 b functioning as a source region and a drain region.

The transistor 300 may be a p-channel transistor or an n-channel transistor.

It is preferable that a region of the semiconductor region 313 where a channel is formed, a region in the vicinity thereof, the low-resistance regions 314 a and 314 b functioning as the source and drain regions, and the like contain a semiconductor such as a silicon-based semiconductor, further preferably single crystal silicon. Alternatively, a material including germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs), gallium aluminum arsenide (GaAlAs), or the like may be contained. Silicon whose effective mass is adjusted by applying stress to the crystal lattice and thereby changing the lattice spacing may be contained. Alternatively, the transistor 300 may be a high-electron-mobility transistor (HEMT) with GaAs and GaAlAs, or the like.

The low-resistance regions 314 a and 314 b contain an element that imparts n-type conductivity, such as arsenic or phosphorus, or an element that imparts p-type conductivity, such as boron, in addition to a semiconductor material used for the semiconductor region 313.

The conductor 316 functioning as a gate electrode can be formed using a semiconductor material such as silicon containing the element that imparts n-type conductivity, such as arsenic or phosphorus, or the element that imparts p-type conductivity, such as boron, or using a conductive material such as a metal material, an alloy material, or a metal oxide material.

Note that the work function of a conductor is determined by a material of the conductor, whereby the threshold voltage can be adjusted. Specifically, it is preferable to use titanium nitride, tantalum nitride, or the like for the conductor. Furthermore, in order to ensure the conductivity and embeddability, it is preferable to use a stacked layer of metal materials such as tungsten and aluminum for the conductor. It is particularly preferable to use tungsten in terms of heat resistance.

Note that the transistor 300 illustrated in FIG. 22 is only an example and is not limited to have the structure illustrated in FIG. 22; a transistor appropriate for a circuit configuration or a driving method can be used.

An insulator 320, an insulator 322, an insulator 324, and an insulator 326 are stacked sequentially so as to cover the transistor 300.

The insulator 320, the insulator 322, the insulator 324, and the insulator 326 can be formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, or aluminum nitride.

The insulator 322 may function as a planarization film for eliminating a level difference caused by the transistor 300 or the like underlying the insulator 322. For example, the top surface of the insulator 322 may be planarized by planarization treatment using a chemical mechanical polishing (CMP) method or the like to increase the level of planarity.

The insulator 324 is preferably formed using a film having a barrier property that prevents hydrogen or impurities from the substrate 311, the transistor 300, or the like from diffusing to a region where the transistor 200 is formed.

As an example of the film having a barrier property against hydrogen, a silicon nitride film formed by a CVD method can be given. The diffusion of hydrogen to a semiconductor element including an oxide semiconductor, such as the transistor 200, degrades the characteristics of the semiconductor element in some cases. Therefore, a film that prevents hydrogen diffusion is preferably provided between the transistor 200 and the transistor 300. The film that prevents hydrogen diffusion is specifically a film from which hydrogen is less likely to be released.

The amount of released hydrogen can be measured by thermal desorption spectroscopy (TDS), for example. The amount of hydrogen released from the insulator 324 that is converted into hydrogen atoms per unit area of the insulator 324 is less than or equal to 10×10¹⁵ atoms/cm², preferably less than or equal to 5×10¹⁵ atoms/cm² in the TDS analysis in the range from 50° C. to 500° C., for example.

Note that the permittivity of the insulator 326 is preferably lower than that of the insulator 324. For example, the dielectric constant of the insulator 326 is preferably lower than 4, further preferably lower than 3. The dielectric constant of the insulator 326 is, for example, preferably 0.7 times or less that of the insulator 324, further preferably 0.6 times or less that of the insulator 324. In the case where a material with a low permittivity is used for an interlayer film, the parasitic capacitance between wirings can be reduced.

A conductor 328, a conductor 330, and the like that are electrically connected to the capacitor 100 or the transistor 200 are provided in the insulator 320, the insulator 322, the insulator 324, and the insulator 326. Note that the conductor 328 and the conductor 330 each function as a plug or a wiring. A plurality of conductors functioning as plugs or wirings are collectively denoted by the same reference numeral in some cases. Furthermore, in this specification and the like, a wiring and a plug electrically connected to the wiring may be a single component. That is, there are cases where part of a conductor serves as a wiring and part of a conductor serves as a plug.

As a material for each of plugs and wirings (e.g., the conductor 328 and the conductor 330), a conductive material such as a metal material, an alloy material, a metal nitride material, or a metal oxide material can be used in a single-layer structure or a stacked-layer structure. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. Alternatively, a low-resistance conductive material such as aluminum or copper is preferably used. The use of a low-resistance conductive material can reduce wiring resistance.

A wiring layer may be provided over the insulator 326 and the conductor 330. For example, in FIG. 22, an insulator 350, an insulator 352, and an insulator 354 are stacked sequentially. Furthermore, a conductor 356 is formed in the insulator 350, the insulator 352, and the insulator 354. The conductor 356 functions as a plug or a wiring. Note that the conductor 356 can be formed using a material similar to those used for forming the conductor 328 and the conductor 330.

Note that the insulator 350 is preferably formed using an insulator having a barrier property against hydrogen, as with the insulator 324, for example. Furthermore, the conductor 356 preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator 350 having a barrier property against hydrogen. In such a structure, the transistor 300 and the transistor 200 can be separated by a barrier layer, so that the diffusion of hydrogen from the transistor 300 to the transistor 200 can be prevented.

Note that as the conductor having a barrier property against hydrogen, tantalum nitride is preferably used, for example. The use of a stack including tantalum nitride and tungsten having high conductivity can inhibit the diffusion of hydrogen from the transistor 300 while the conductivity of a wiring is ensured. In this case, a tantalum nitride layer having a barrier property against hydrogen is preferably in contact with the insulator 350 having a barrier property against hydrogen.

A wiring layer may be provided over the insulator 354 and the conductor 356. For example, in FIG. 22, an insulator 360, an insulator 362, and an insulator 364 are stacked sequentially. Furthermore, a conductor 366 is formed in the insulator 360, the insulator 362, and the insulator 364. The conductor 366 functions as a plug or a wiring. Note that the conductor 366 can be formed using a material similar to those used for forming the conductor 328 and the conductor 330.

Note that the insulator 360 is preferably formed using an insulator having a barrier property against hydrogen, as with the insulator 324, for example. Furthermore, the conductor 366 preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator 360 having a barrier property against hydrogen. In such a structure, the transistor 300 and the transistor 200 can be separated by a barrier layer, so that the diffusion of hydrogen from the transistor 300 to the transistor 200 can be prevented.

A wiring layer may be provided over the insulator 364 and the conductor 366. For example, in FIG. 22, an insulator 370, an insulator 372, and an insulator 374 are stacked sequentially. Furthermore, a conductor 376 is formed in the insulator 370, the insulator 372, and the insulator 374. The conductor 376 functions as a plug or a wiring. Note that the conductor 376 can be formed using a material similar to those used for forming the conductor 328 and the conductor 330.

Note that the insulator 370 is preferably formed using an insulator having a barrier property against hydrogen, as with the insulator 324, for example. Furthermore, the conductor 376 preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator 370 having a barrier property against hydrogen. In such a structure, the transistor 300 and the transistor 200 can be separated by a barrier layer, so that the diffusion of hydrogen from the transistor 300 to the transistor 200 can be prevented.

A wiring layer may be provided over the insulator 374 and the conductor 376. For example, in FIG. 22, an insulator 380, an insulator 382, and an insulator 384 are stacked sequentially. Furthermore, a conductor 386 is formed in the insulator 380, the insulator 382, and the insulator 384. The conductor 386 functions as a plug or a wiring. Note that the conductor 386 can be formed using a material similar to those used for forming the conductor 328 and the conductor 330.

Note that the insulator 380 is preferably formed using an insulator having a barrier property against hydrogen, as with the insulator 324, for example. Furthermore, the conductor 386 preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening of the insulator 380 having a barrier property against hydrogen. In such a structure, the transistor 300 and the transistor 200 can be separated by a barrier layer, so that the diffusion of hydrogen from the transistor 300 to the transistor 200 can be prevented.

An insulator 210, an insulator 212, an insulator 214, and an insulator 216 are stacked sequentially over the insulator 384. A material having a barrier property against oxygen or hydrogen is preferably used for any of the insulator 210, the insulator 212, the insulator 214, and the insulator 216.

Each of the insulators 210 and 214 is preferably formed using, for example, a film having a barrier property that prevents hydrogen or impurities from the substrate 311, a region where the transistor 300 is formed, or the like from diffusing to a region where the transistor 200 is formed. Thus, each of the insulators 210 and 214 can be formed using a material similar to that used for forming the insulator 324.

As an example of the film having a barrier property against hydrogen, a silicon nitride film formed by a CVD method can be given. The diffusion of hydrogen to a semiconductor element including an oxide semiconductor, such as the transistor 200, degrades the characteristics of the semiconductor element in some cases. Therefore, a film that prevents hydrogen diffusion is preferably provided between the transistor 200 and the transistor 300. The film that prevents hydrogen diffusion is specifically a film from which hydrogen is less likely to be released.

For the film having a barrier property against hydrogen used for each of the insulators 210 and 214, for example, a metal oxide such as aluminum oxide, hafnium oxide, or tantalum oxide is preferably used.

In particular, aluminum oxide has an excellent blocking effect that prevents penetration of oxygen and impurities such as hydrogen or moisture that cause a change in electrical characteristics of the transistor. Accordingly, the use of aluminum oxide can prevent the entry of impurities such as hydrogen or moisture into the transistor 200 in and after a manufacturing process of the transistor. In addition, release of oxygen from the oxide contained in the transistor 200 can be prevented. Therefore, aluminum oxide is suitably used for a protective film of the transistor 200.

The insulators 212 and 216 can be formed using a material similar to that used for forming the insulator 320, for example. Interlayer films formed of a material with a relatively low permittivity can reduce the parasitic capacitance between wirings. For example, a silicon oxide film, a silicon oxynitride film, or the like can be used for the insulators 212 and 216.

A conductor 218, a conductor (e.g., an electrode serving as a back gate) included in the transistor 200, and the like are provided in the insulators 210, 212, 214, and 216. Note that the conductor 218 functions as a plug or a wiring that is electrically connected to the capacitor 100 or the transistor 300. The conductor 218 can be formed using a material similar to those used for forming the conductors 328 and 330.

Note that the electrode serving as the back gate of the transistor 200 and a wiring to supply a potential to the electrode may be formed in the same tier, as illustrated in FIG. 22. Alternatively, as described in the above embodiment, the wiring to supply a potential to the electrode may be formed in a tier under the electrode serving as the back gate of the transistor 200 to constitute a stack. In that case, the insulator 214, the insulator 216, and the like are formed to have stacked-layer structures appropriately in accordance with the structure described in the above embodiment.

In particular, part of the conductor 218 that is in contact with the insulators 210 and 214 is preferably a conductor having a barrier property against oxygen, hydrogen, and water. In such a structure, the transistors 300 and 200 can be completely separated by the layer having a barrier property against oxygen, hydrogen, and water. As a result, the diffusion of hydrogen from the transistor 300 to the transistor 200 can be prevented.

The transistor 200 is provided over the insulator 216. Note that any of the transistors included in the semiconductor devices described in the above embodiment can be used as the transistor 200. As the transistor 200, for example, the transistor 1000, the transistor 1000 a, or the like can be used. FIG. 22 illustrates an example where the transistor 1000 is used as the transistor 200. Note that the transistor 200 in FIG. 22 is only an example and is not limited to have the structure illustrated in FIG. 22; a transistor appropriate for a circuit configuration or a driving method can be used.

An insulator 280 is provided over the transistor 200. In the insulator 280, an excess-oxygen region is preferably formed. In particular, in the case where an oxide semiconductor is used in the transistor 200 and an insulator having an excess-oxygen region is provided in an interlayer film or the like in the vicinity of the transistor 200, oxygen vacancies in the oxide 406 included in the transistor 200 are reduced, whereby the reliability can be improved. The insulator 280 that covers the transistor 200 may function as a planarization film that covers a roughness thereunder. Note that the insulator 280 is provided to be in contact with an insulator 225 that is formed over the transistor 200.

For the insulator having an excess-oxygen region, specifically, an oxide that releases part of oxygen by heating is preferably used. The oxide that releases oxygen by heating is an oxide film in which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C.

As such a material, a material containing silicon oxide or silicon oxynitride is preferably used, for example. Alternatively, a metal oxide can be used. Note that in this specification, “silicon oxynitride” refers to a material that contains oxygen at a higher proportion than nitrogen, and “silicon nitride oxide” refers to a material that contains nitrogen at a higher proportion than oxygen.

An insulator 282 may be provided over the insulator 280. A material having a barrier property against oxygen or hydrogen is preferably used for the insulator 282. Thus, the insulator 282 can be formed using a material similar to that used for forming the insulator 214. For the insulator 282, a metal oxide such as aluminum oxide, hafnium oxide, or tantalum oxide is preferably used, for example. When the insulator 282 is deposited by a sputtering method with the use of plasma that contains oxygen, oxygen can be added to the insulator 280 that serves as a base layer of the oxide.

In particular, aluminum oxide has an excellent blocking effect that prevents penetration of oxygen and impurities such as hydrogen or moisture that cause a change in electrical characteristics of the transistor. Accordingly, the use of aluminum oxide can prevent the entry of impurities such as hydrogen or moisture into the transistor 200 in and after a manufacturing process of the transistor. In addition, release of oxygen from the oxide contained in the transistor 200 can be prevented. Therefore, aluminum oxide is suitably used for a protective film of the transistor 200.

In the case where the transistor 1000 is provided as the transistor 200, the insulator 214 corresponds to the insulator 401 or the insulator 432, the conductor 218 corresponds to the conductor 310 or the conductor 440, the insulator 216 corresponds to the insulator 430 or the insulator 301, an insulator 220 corresponds to the insulator 302, an insulator 222 corresponds to the insulator 303, an insulator 224 corresponds to the insulator 402, the insulator 225 corresponds to the insulator 409, and the insulator 280 corresponds to the insulator 415. Thus, the description of the corresponding components in the above embodiment can be referred to.

An insulator 286 is provided over the insulator 282. The insulator 286 can be formed using a material similar to that used for forming the insulator 320. In the case where an interlayer film formed of a material with a relatively low permittivity is used for the insulator, the parasitic capacitance between wirings can be reduced. For example, a silicon oxide film, a silicon oxynitride film, or the like can be used for the insulator 286.

A conductor 246, a conductor 248, and the like are provided in the insulators 220, 222, 224, 280, 282, and 286.

The conductors 246 and 248 function as plugs or wirings that are electrically connected to the capacitor 100, the transistor 200, or the transistor 300. The conductors 246 and 248 can be formed using a material similar to those used for forming the conductors 328 and 330.

The capacitor 100 is provided above the transistor 200. The capacitor 100 includes a conductor 110, a conductor 120, and an insulator 130.

A conductor 112 may be provided over the conductors 246 and 248. The conductor 112 functions as a plug or a wiring that is electrically connected to the capacitor 100, the transistor 200, or the transistor 300. The conductor 110 functions as the electrode of the capacitor 100. The conductor 112 and the conductor 110 can be formed at the same time.

The conductor 112 and the conductor 110 can be formed using a metal film containing an element selected from molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, and scandium; a metal nitride film containing any of the above elements as its component (a tantalum nitride film, a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film); or the like. Alternatively, it is possible to use a conductive material such as an indium tin oxide, an indium oxide containing tungsten oxide, an indium zinc oxide containing tungsten oxide, an indium oxide containing titanium oxide, an indium tin oxide containing titanium oxide, an indium zinc oxide, or an indium tin oxide to which silicon oxide is added.

The conductor 112 and the conductor 110 each have a single-layer structure in FIG. 22; however, one embodiment of the present invention is not limited thereto, and a stacked-layer structure of two or more layers may be used. For example, between a conductor having a barrier property and a conductor having high conductivity, a conductor that is highly adhesive to the conductor having a barrier property and the conductor having high conductivity may be formed.

As a dielectric of the capacitor 100, the insulator 130 is provided over the conductors 112 and 110. The insulator 130 can be formed to have a single-layer structure or a stacked-layer structure using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitride oxide, hafnium nitride, or the like.

A material with high dielectric strength, such as silicon oxynitride, is preferably used for the insulator 130, for example. In the capacitor 100 having such a structure, the dielectric strength can be increased and the electrostatic breakdown of the capacitor 100 can be prevented because of the insulator 130.

Over the insulator 130, the conductor 120 is provided so as to overlap with the conductor 110. Note that the conductor 120 can be formed using a conductive material such as a metal material, an alloy material, or a metal oxide material. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. In the case where the conductor 120 is formed concurrently with another component such as a conductor, copper (Cu), aluminum (Al), or the like that is a low-resistance metal material can be used.

An insulator 150 is provided over the conductor 120 and the insulator 130. The insulator 150 can be formed using a material similar to that used for forming the insulator 320. The insulator 150 may function as a planarization film that covers a roughness thereunder.

The above is the description of the structure example. With the use of this structure, in a semiconductor device including a transistor containing an oxide semiconductor, a change in electrical characteristics can be inhibited and reliability can be improved, power consumption can be reduced, or miniaturization or high integration of the semiconductor device can be achieved. Alternatively, a miniaturized or highly integrated semiconductor device can be provided with high productivity.

Modification Example 1 of Memory Device 1

A modification example of this embodiment is illustrated in FIG. 23. FIG. 23 is different from FIG. 22 in the structure of the transistor 300.

In the transistor 300 illustrated in FIG. 23, the semiconductor region 313 (part of the substrate 311) in which a channel is formed has a protruding portion. Furthermore, the conductor 316 is provided to cover the top and side surfaces of the semiconductor region 313 with the insulator 315 positioned therebetween. Note that the conductor 316 may be formed using a material for adjusting the work function. The transistor 300 having such a structure is also referred to as a FIN transistor because the protruding portion of the semiconductor substrate is utilized. An insulator functioning as a mask for forming the protruding portion may be provided in contact with the top surface of the protruding portion. Although the case where the protruding portion is formed by processing part of the semiconductor substrate is described here, a semiconductor film having a protruding shape may be formed by processing an SOI substrate.

The above is the description of the modification example. With the use of this structure, in a semiconductor device including a transistor containing an oxide semiconductor, a change in electrical characteristics can be inhibited and reliability can be improved, power consumption can be reduced, or miniaturization or high integration of the semiconductor device can be achieved. Alternatively, a miniaturized or highly integrated semiconductor device can be provided with high productivity.

Modification Example 2 of Memory Device 1

Another modification example of this embodiment is illustrated in FIG. 24. FIG. 24 is different from FIG. 22 in the structure of the capacitor 100.

In a memory device illustrated in FIG. 24, an insulator 287 is provided over the insulator 286, the conductor 112 is embedded in the insulator 287, an insulator 155 is provided over the insulator 287, the conductors 110 are provided in a plurality of openings formed in the insulator 155, the insulator 130 is provided over the conductors 110, and the conductor 120 is provided over the insulator 130 so as to overlap with the conductors 110. The conductor 112 is provided to connect the conductor 248 electrically connected to the transistor 200 and the conductor 248 electrically connected to the transistor 300, and the conductors 110 are provided in contact with the conductor 112. The insulator 287 and the insulator 155 can be formed using a material similar to that of the insulator 320.

In the capacitor 100 illustrated in FIG. 24, the conductors 110, the insulator 130, and the conductor 120 overlap with each other in the openings formed in the insulator 155; thus, the conductors 110, the insulator 130, and the conductor 120 preferably have favorable coverage. For this reason, the conductors 110, the insulator 130, and the conductor 120 are preferably deposited by a method with which a film having favorable step coverage can be formed, such as a CVD method or an ALD method.

Because the capacitor 100 is formed along the shapes of the openings formed in the insulator 155, the capacitance can be larger as the openings become deeper. Furthermore, the capacitance can be larger as the number of the openings becomes larger. With the capacitor 100 having such a structure, the capacitance can be increased without increasing the area of the top surface of the capacitor 100.

The above is the description of the modification example. With the use of this structure, in a semiconductor device including a transistor containing an oxide semiconductor, a change in electrical characteristics can be inhibited and reliability can be improved, power consumption can be reduced, or miniaturization or high integration of the semiconductor device can be achieved. Alternatively, a miniaturized or highly integrated semiconductor device can be provided with high productivity.

<Structure of Memory Cell Array>

FIG. 25 illustrates an example of a memory cell array of this embodiment. When the memory devices each of which is illustrated in FIG. 22 or FIG. 23 are arranged as memory cells in a matrix, a memory cell array can be formed. FIG. 25 is a cross-sectional view that illustrates part of a row in which the memory devices each of which is illustrated in FIG. 22 are arranged in a matrix.

In a memory device illustrated in FIG. 25, a memory cell 600 a and a memory cell 600 b are arranged adjacent to each other. As in the memory device illustrated in FIG. 22, the transistors 300 and 200 and the capacitor 100 are included and electrically connected to the wirings 3001, 3002, 3003, 3004, 3005, and 3006 in each of the memory cells 600 a and 600 b. Also in the memory cells 600 a and 600 b, a node where a gate of the transistor 300 and one electrode of the capacitor 100 are electrically connected to each other is referred to as the node FG. Note that the wiring 3002 is shared by the memory cells 600 a and 600 b adjacent to each other.

Note that in the case where memory cells are arrayed, it is necessary that data of a desired memory cell be read in read operation. For example, in the case of a NOR-type memory cell array, only data of a desired memory cell can be read by turning off the transistors 300 of memory cells from which data is not read. In this case, a potential at which the transistor 300 is turned off regardless of the charge supplied to the node FG, that is, a potential lower than V_(th) _(_) _(H), is supplied to the wiring 3005 connected to the memory cells from which data is not read. Alternatively, in the case of a NAND-type memory cell array, for example, only data of a desired memory cell can be read by turning on the transistors 300 of memory cells from which data is not read. In this case, a potential at which the transistor 300 is turned on regardless of the charge supplied to the node FG, that is, a potential higher than V_(th) _(_) _(L), is supplied to the wiring 3005 connected to the memory cells from which data is not read.

The above is the description of the structure examples. With the use of any of the structures, in a semiconductor device including a transistor containing an oxide semiconductor, a change in electrical characteristics can be prevented and reliability can be improved, power consumption can be reduced, or miniaturization or high integration of the semiconductor device can be achieved. Alternatively, a miniaturized or highly integrated semiconductor device can be provided with high productivity.

The structures, methods, and the like described in this embodiment can be combined with any of the structures, methods, and the like described in the other embodiments as appropriate.

Embodiment 3

In this embodiment, a frame memory including a semiconductor device of one embodiment of the present invention, which can be used in a display controller IC, a source driver IC, or the like, is described.

A dynamic random access memory (DRAM) including memory cells of 1T1C (one transistor, one capacitor) type can be used as the frame memory, for example. A memory device in which OS transistors are used in memory cells (hereinafter referred to as an OS memory) can also be used. Here, a RAM including memory cells of 1T1C type is described as an example of the OS memory. Such a RAM is herein referred to as a dynamic oxide semiconductor RAM (DOSRAM). FIG. 26 illustrates a configuration example of a DOSRAM.

<<DOSRAM 1400>>

A DOSRAM 1400 includes a controller 1405, a row circuit 1410, a column circuit 1415, and a memory cell and sense amplifier array 1420 (hereinafter referred to as MC-SA array 1420).

The row circuit 1410 includes a decoder 1411, a word line driver circuit 1412, a column selector 1413, and a sense amplifier driver circuit 1414. The column circuit 1415 includes a global sense amplifier array 1416 and an input/output circuit 1417. The global sense amplifier array 1416 includes a plurality of global sense amplifiers 1447. The MC-SA array 1420 includes a memory cell array 1422, a sense amplifier array 1423, and global bit lines GBLL and GBLR.

(MC-SA Array 1420)

The MC-SA array 1420 has a stacked-layer structure where the memory cell array 1422 is stacked over the sense amplifier array 1423. The global bit lines GBLL and GBLR are stacked over the memory cell array 1422. The DOSRAM 1400 adopts a hierarchical bit line structure, where the bit lines are layered into local and global bit lines.

The memory cell array 1422 includes N local memory cell arrays 1425<0> to 1425<N−1>, where N is an integer greater than or equal to 2. FIG. 27A illustrates a configuration example of the local memory cell array 1425. The local memory cell array 1425 includes a plurality of memory cells 1445, a plurality of word lines WL, and a plurality of bit lines BLL and BLR. In the example in FIG. 27A, the local memory cell array 1425 has an open bit-line architecture but may have a folded bit-line architecture.

FIG. 27B illustrates a circuit configuration example of the memory cell 1445. The memory cell 1445 includes a transistor MW1, a capacitor CS1, and terminals B1 and B2. The transistor MW1 has a function of controlling the charging and discharging of the capacitor CS1. A gate of the transistor MW1 is electrically connected to the word line, a first terminal of the transistor MW1 is electrically connected to the bit line, and a second terminal of the transistor MW1 is electrically connected to a first terminal of the capacitor CS1. A second terminal of the capacitor CS1 is electrically connected to the terminal B2. A constant voltage (e.g., low power supply voltage) is input to the terminal B2.

The transistor MW1 includes a back gate, and the back gate is electrically connected to the terminal B1. This makes it possible to change the threshold voltage of the transistor MW1 with a voltage applied to the terminal B1. For example, a fixed voltage (e.g., negative constant voltage) may be applied to the terminal B1; alternatively, the voltage applied to the terminal B1 may be changed in response to the operation of the DOSRAM 1400.

The back gate of the transistor MW1 may be electrically connected to the gate, the source, or the drain of the transistor MW1. Alternatively, the transistor MW1 does not necessarily include the back gate.

The sense amplifier array 1423 includes N local sense amplifier arrays 1426<0> to 1426<N−1>. The local sense amplifier array 1426 includes one switch array 1444 and a plurality of sense amplifiers 1446. A bit line pair is electrically connected to the sense amplifier 1446. The sense amplifier 1446 has a function of precharging the bit line pair, a function of amplifying a voltage difference between the bit line pair, and a function of retaining the voltage difference. The switch array 1444 has a function of selecting a bit line pair and electrically connecting the selected bit line pair and a global bit line pair to each other.

Here, two bit lines that are compared simultaneously by the sense amplifier are collectively referred to as the bit line pair. Two global bit lines that are compared simultaneously by the global sense amplifier are collectively referred to as the global bit line pair. The bit line pair can be referred to as a pair of bit lines, and the global bit line pair can be referred to as a pair of global bit lines. Here, a bit line BLL and a bit line BLR form one bit line pair. A global bit line GBLL and a global bit line GBLR form one global bit line pair. In the description hereinafter, the expressions “bit line pair (BLL, BLR)” and “global bit line pair (GBLL, GBLR)” are also used.

(Controller 1405)

The controller 1405 has a function of controlling the overall operation of the DOSRAM 1400. The controller 1405 has a function of performing logic operation on a command signal that is input from the outside and determining an operation mode, a function of generating control signals for the row circuit 1410 and the column circuit 1415 so that the determined operation mode is executed, a function of retaining an address signal that is input from the outside, and a function of generating an internal address signal.

(Row Circuit 1410)

The row circuit 1410 has a function of driving the MC-SA array 1420. The decoder 1411 has a function of decoding an address signal. The word line driver circuit 1412 generates a selection signal for selecting the word line WL of a row that is to be accessed.

The column selector 1413 and the sense amplifier driver circuit 1414 are circuits for driving the sense amplifier array 1423. The column selector 1413 has a function of generating a selection signal for selecting the bit line of a column that is to be accessed. The selection signal from the column selector 1413 controls the switch array 1444 of each local sense amplifier array 1426. The control signal from the sense amplifier driver circuit 1414 drives each of the plurality of local sense amplifier arrays 1426 independently.

(Column Circuit 1415)

The column circuit 1415 has a function of controlling the input of data signals WDA[31:0], and a function of controlling the output of data signals RDA[31:0]. The data signals WDA[31:0] are write data signals, and the data signals RDA[31:0] are read data signals.

The global sense amplifier 1447 is electrically connected to the global bit line pair (GBLL, GBLR). The global sense amplifier 1447 has a function of amplifying a voltage difference between the global bit line pair (GBLL, GBLR), and a function of retaining the voltage difference. Data are written to and read from the global bit line pair (GBLL, GBLR) by the input/output circuit 1417.

A write operation of the DOSRAM 1400 is briefly described. Data are written to the global bit line pair by the input/output circuit 1417. The data of the global bit line pair are retained by the global sense amplifier array 1416. By the switch array 1444 of the local sense amplifier array 1426 specified by the address signal, the data of the global bit line pair are written to the bit line pair of the column where data are to be written. The local sense amplifier array 1426 amplifies the written data, and then retains the amplified data. In the specified local memory cell array 1425, the word line WL of the row where data are to be written is selected by the row circuit 1410, and the data retained at the local sense amplifier array 1426 are written to the memory cell 1445 of the selected row.

A read operation of the DOSRAM 1400 is briefly described. One row of the local memory cell array 1425 is specified with the address signal. In the specified local memory cell array 1425, the word line WL of the row where data are to be read is selected, and data of the memory cell 1445 are written to the bit line. The local sense amplifier array 1426 detects a voltage difference between the bit line pair of each column as data, and retains the data. The switch array 1444 writes the data of a column specified by the address signal to the global bit line pair; the data are chosen from the data retained at the local sense amplifier array 1426. The global sense amplifier array 1416 detects and retains the data of the global bit line pair. The data retained at the global sense amplifier array 1416 are output to the input/output circuit 1417. Thus, the read operation is completed.

The DOSRAM 1400 has no limitations on the number of rewrites in principle and data can be read and written with low energy consumption, because data are rewritten by charging and discharging the capacitor CS1. Simple circuit configuration of the memory cell 1445 allows a high memory capacity.

The transistor MW1 is an OS transistor. The extremely low off-state current of the OS transistor can inhibit leakage of charge from the capacitor CS1. Therefore, the retention time of the DOSRAM 1400 is considerably longer than that of DRAM. This allows less frequent refresh, which can reduce power needed for refresh operations. For this reason, the DOSRAM 1400 used as the frame memory can reduce the power consumption of the display controller IC and the source driver IC.

Since the MC-SA array 1420 has a stacked-layer structure, the bit line can be shortened to a length that is close to the length of the local sense amplifier array 1426. A shorter bit line results in smaller bit line capacitance, which allows the storage capacitance of the memory cell 1445 to be reduced. In addition, providing the switch array 1444 in the local sense amplifier array 1426 allows the number of long bit lines to be reduced. For the reasons described above, a load to be driven during access to the DOSRAM 1400 is reduced, enabling a reduction in the energy consumption of the display controller IC and the source driver IC.

The structure described in this embodiment can be used in appropriate combination with any of the structures and methods described in the other embodiments.

Embodiment 4

In this embodiment, a field-programmable gate array (FPGA) is described as an example of a semiconductor device in which a transistor whose semiconductor includes an oxide (OS transistor) is used. In an FPGA of this embodiment, an OS memory is used for a configuration memory and a register. Here, such an FPGA is referred to as an “OS-FPGA”.

The OS memory is a memory including at least a capacitor and an OS transistor that controls charge and discharge of the capacitor. The OS memory has excellent retention characteristics because the OS transistor has an extremely low off-state current and thus can function as a nonvolatile memory.

FIG. 28A illustrates a configuration example of an OS-FPGA. An OS-FPGA 3110 illustrated in FIG. 28A is capable of normally-off (NOFF) computing for context switching by a multi-context configuration and fine-grained power gating in each PLE. The OS-FPGA 3110 includes a controller 3111, a word driver 3112, a data driver 3113, and a programmable area 3115.

The programmable area 3115 includes two input/output blocks (IOBs) 3117 and a core 3119. The IOB 3117 includes a plurality of programmable input/output circuits. The core 3119 includes a plurality of logic array blocks (LABs) 3120 and a plurality of switch array blocks (SABs) 3130. The LAB 3120 includes a plurality of PLEs 3121. FIG. 28B illustrates an example in which the LAB 3120 includes five PLEs 3121. As illustrated in FIG. 28C, the SAB 3130 includes a plurality of switch blocks (SBs) 3131 arranged in array. The LAB 3120 is connected to the LABs 3120 in four directions (on the left, right, top, and bottom sides) through its input terminals and the SABs 3130.

The SB 3131 is described with reference to FIGS. 29A to 29C. To the SB 3131 in FIG. 29A, data, datab, signals context[1:0], and signals word[1:0] are input. The data and the datab are configuration data, and the logics of the data and the datab are complementary to each other. The number of contexts in the OS-FPGA 3110 is two, and the signals context[1:0] are context selection signals. The signals word[1:0] are word line selection signals, and wirings to which the signals word[1:0] are input are each a word line.

The SB 3131 includes a programmable routing switch (PRS) 3133[0] and a PRS 3133[1]. The PRS 3133[0] and the PRS 3133[1] each include a configuration memory (CM) that can store complementary data. Note that in the case where the PRS 3133[0] and the PRS 3133[1] are not distinguished from each other, they are each referred to as a PRS 3133. The same applies to other elements.

FIG. 29B illustrates a circuit configuration example of the PRS 3133[0]. The PRS 3133[0] and the PRS 3133[1] have the same circuit configuration. The PRS 3133[0] and the PRS 3133[1] are different from each other in a context selection signal and a word line selection signal that are input. The signal context[0] and the signal word[0] are input to the PRS 3133[0], and the signal context[1] and the signal word[1] are input to the PRS 3133[1]. For example, in the SB 3131, when the signal context[0] is set to “H”, the PRS 3133[0] is activated.

The PRS 3133[0] includes a CM 3135 and a Si transistor M31. The Si transistor M31 is a pass transistor that is controlled by the CM 3135. The CM 3135 includes a memory circuit 3137 and a memory circuit 3137B. The memory circuit 3137 and the memory circuit 3137B have the same circuit configuration. The memory circuit 3137 includes a capacitor C31, an OS transistor MO31, and an OS transistor MO32. The memory circuit 3137B includes a capacitor CB31, an OS transistor MOB31, and an OS transistor MOB32.

The OS transistors MO31, MO32, MOB31, and MOB32 each include a back gate, and these back gates are electrically connected to power supply lines that each supply a fixed voltage.

A gate of the Si transistor M31, a gate of the OS transistor MO32, and a gate of the OS transistor MOB32 correspond to a node N31, a node N32, and a node NB32, respectively. The node 32 and the node NB32 are each a charge retention node of the CM 3135. The OS transistor MO32 controls the conduction state between the node N31 and a signal line for the signal context[0]. The OS transistor MOB32 controls the conduction state between the node N31 and a low-potential power supply line VSS.

A logic of data that the memory circuit 3137 retains and a logic of data that the memory circuit 3137B retains are complementary to each other. Thus, either the OS transistor MO32 or the OS transistor MOB32 is turned on.

The operation example of the PRS 3133[0] is described with reference to FIG. 29C. In the PRS 3133[0], in which configuration data has already been written, the node N32 of the PRS 3133[0] is at “H”, whereas the node NB32 is at “L”.

The PRS 3133[0] is inactivated while the signal context[0] is at “L”. During this period, even when an input terminal of the PRS 3133[0] is transferred to “H”, the gate of the Si transistor M31 is kept at “L” and an output terminal of the PRS 3133[0] is also kept at “L”.

The PRS 3133[0] is activated while the signal context[0] is at “H”. When the signal context[0] is transferred to “H”, the gate of the Si transistor M31 is transferred to “H” by the configuration data stored in the CM 3135.

While the PRS 3133[0] is active, when the potential of the input terminal is changed to “H”, the gate voltage of the Si transistor M31 is increased by boosting because the OS transistor MO32 of the memory circuit 3137 is a source follower. As a result, the OS transistor MO32 of the memory circuit 3137 loses the driving capability, and the gate of the Si transistor M31 is brought into a floating state.

In the PRS 3133 with a multi-context function, the CM 3135 also functions as a multiplexer.

FIG. 30 illustrates a configuration example of the PLE 3121. The PLE 3121 includes a lookup table (LUT) block 3123, a register block 3124, a selector 3125, and a CM 3126. The LUT block 3123 is configured to select and output data in the LUT block in accordance with inputs inA to inD. The selector 3125 selects an output of the LUT block 3123 or an output of the register block 3124 in accordance with the configuration data stored in the CM 3126.

The PLE 3121 is electrically connected to a power supply line for a voltage VDD through a power switch 3127. Whether the power switch 3127 is turned on or off is determined in accordance with configuration data stored in a CM 3128. Fine-grained power gating can be performed by providing the power switch 3127 for each PLE 3121. The PLE 3121 that is not used after context switching can be power gated owing to the fine-grained power gating function; thus, standby power can be effectively reduced.

The register block 3124 is formed by nonvolatile registers to achieve normally-off computing. The nonvolatile registers in the PLE 3121 are each a flip-flop provided with an OS memory (hereinafter referred to as OS-FF).

The register block 3124 includes an OS-FF 3140[1] and an OS-FF 3140[2]. A signal user_res, a signal load, and a signal store are input to the OS-FF 3140[1] and the OS-FF 3140[2]. A clock signal CLK1 is input to the OS-FF 3140[1] and a clock signal CLK2 is input to the OS-FF 3140[2]. FIG. 31A illustrates a configuration example of the OS-FF 3140.

The OS-FF 3140 includes a FF 3141 and a shadow register 3142. The FF 3141 includes a node CK, a node R, a node D, a node Q, and a node QB. A clock signal is input to the node CK. The signal user res is input to the node R. The signal user_res is a reset signal. The node D is a data input node, and the node Q is a data output node. The logics of the node Q and the node QB are complementary to each other.

The shadow register 3142 can function as a backup circuit of the FF 3141. The shadow register 3142 backs up data of the node Q and data of the node QB in response to the signal store and writes back the backed up data to the node Q and the node QB in response to the signal load.

The shadow register 3142 includes an inverter circuit 3188, an inverter circuit 3189, a Si transistor M37, a Si transistor MB37, a memory circuit 3143, and a memory circuit 3143B. The memory circuit 3143 and the memory circuit 3143B each have the same circuit configuration as the memory circuit 3137 of the PRS 3133. The memory circuit 3143 includes a capacitor C36, an OS transistor MO35, and an OS transistor MO36. The memory circuit 3143B includes a capacitor CB36, an OS transistor MOB35, and an OS transistor MOB36. A node N36 and a node NB36 correspond to a gate of the OS transistor MO36 and a gate of the OS transistor MOB36, respectively, and are each a charge retention node. A node N37 and a node NB37 correspond to a gate of the Si transistor M37 and a gate of the Si transistor MB37, respectively.

The OS transistors MO35, MO36, MOB35, and MOB36 each include a back gate, and these back gates are electrically connected to power supply lines that each supply a fixed voltage.

An example of an operation method of the OS-FF 3140 is described with reference to FIG. 31B.

(Backup)

When the signal store at “H” is input to the OS-FF 3140, the shadow register 3142 backs up data of the FF 3141. The node N36 becomes “L” when the data of the node Q is written thereto, and the node NB36 becomes “H” when the data of the node QB is written thereto. After that, power gating is performed and the power switch 3127 is turned off. Although the data of the node Q and the data of the node QB of the FF 3141 are lost, the shadow register 3142 retains the backed up data even when power supply is stopped.

(Recovery)

The power switch 3127 is turned on to supply power to the PLE 3121. After that, when the signal load at “H” is input to the OS-FF 3140, the shadow register 3142 writes back the backed up data to the FF 3141. The node N37 is kept at “L” because the node N36 is at “L”, and the node NB37 becomes “H” because the node NB36 is at “H”. Thus, the node Q becomes “H” and the node QB becomes “L”. That is, the OS-FF 3140 is restored to a state at the backup operation.

A combination of the fine-grained power gating and backup/recovery operation of the OS-FF 3140 allows power consumption of the OS-FPGA 3110 to be effectively reduced.

A possible error in a memory circuit is a soft error due to the entry of radiation. The soft error is a phenomenon in which a malfunction such as inversion of data stored in a memory is caused by electron-hole pair generation when a transistor is irradiated with a rays emitted from a material of a memory or a package or the like, secondary cosmic ray neutrons generated by nuclear reaction of primary cosmic rays entering the Earth's atmosphere from outer space with nuclei of atoms existing in the atmosphere, or the like. An OS memory including an OS transistor has a high soft-error tolerance. Therefore, the OS-FPGA 3110 including an OS memory can have high reliability.

The structure described in this embodiment can be used in appropriate combination with any of the structures and methods described in the other embodiments.

Embodiment 5

In this embodiment, an example of a CPU including the semiconductor device of one embodiment of the present invention, such as the above-described memory device, is described.

<Configuration of CPU>

A semiconductor device 5400 shown in FIG. 32 includes a CPU core 5401, a power management unit 5421, and a peripheral circuit 5422. The power management unit 5421 includes a power controller 5402 and a power switch 5403. The peripheral circuit 5422 includes a cache 5404 including cache memory, a bus interface (BUS I/F) 5405, and a debug interface (Debug I/F) 5406. The CPU core 5401 includes a data bus 5423, a control unit 5407, a PC (program counter) 5408, a pipeline register 5409, a pipeline register 5410, an ALU (arithmetic logic unit) 5411, and a register file 5412. Data is transmitted between the CPU core 5401 and the peripheral circuit 5422 such as the cache 5404 via the data bus 5423.

The semiconductor device (cell) can be used for many logic circuits typified by the power controller 5402 and the control unit 5407, particularly for all logic circuits that can be constituted using standard cells. Accordingly, the semiconductor device 5400 can be small. The semiconductor device 5400 can have reduced power consumption. The semiconductor device 5400 can have a higher operating speed. The semiconductor device 5400 can have a smaller power supply voltage variation.

When p-channel Si transistors and the transistor described in the above embodiment that includes an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region are used in the semiconductor device (cell) and the semiconductor device (cell) is used in the semiconductor device 5400, the semiconductor device 5400 can be small. The semiconductor device 5400 can have reduced power consumption. The semiconductor device 5400 can have a higher operating speed. Particularly when the Si transistors are only p-channel ones, the manufacturing cost can be reduced.

The control unit 5407 has functions of decoding and executing instructions contained in a program such as inputted applications by controlling the overall operations of the PC 5408, the pipeline registers 5409 and 5410, the ALU 5411, the register file 5412, the cache 5404, the bus interface 5405, the debug interface 5406, and the power controller 5402.

The ALU 5411 has a function of performing a variety of arithmetic operations such as four arithmetic operations and logic operations.

The cache 5404 has a function of temporarily storing frequently used data. The PC 5408 is a register having a function of storing an address of an instruction to be executed next. Note that although not shown in FIG. 32, the cache 5404 is provided with a cache controller for controlling the operation of the cache memory.

The pipeline register 5409 has a function of temporarily storing instruction data.

The register file 5412 includes a plurality of registers including a general purpose register and can store data that is read from the main memory, data obtained as a result of arithmetic operations in the ALU 5411, or the like.

The pipeline register 5410 has a function of temporarily storing data used for arithmetic operations of the ALU 5411, data obtained as a result of arithmetic operations of the ALU 5411, or the like.

The bus interface 5405 has a function of a path for data between the semiconductor device 5400 and various devices outside the semiconductor device 5400. The debug interface 5406 has a function of a path of a signal for inputting an instruction to control debugging to the semiconductor device 5400.

The power switch 5403 has a function of controlling supply of a power supply voltage to various circuits included in the semiconductor device 5400 other than the power controller 5402. The above various circuits belong to several different power domains. The power switch 5403 controls whether the power supply voltage is supplied to the various circuits in the same power domain. In addition, the power controller 5402 has a function of controlling the operation of the power switch 5403.

The semiconductor device 5400 having the above structure is capable of performing power gating. A description is given of an example of the power gating operation sequence.

First, by the CPU core 5401, timing for stopping the supply of the power supply voltage is set in a register of the power controller 5402. Then, an instruction of starting power gating is sent from the CPU core 5401 to the power controller 5402. Then, various registers and the cache 5404 included in the semiconductor device 5400 start data saving. Then, the power switch 5403 stops the supply of a power supply voltage to the various circuits other than the power controller 5402 included in the semiconductor device 5400. Then, an interrupt signal is input to the power controller 5402, whereby the supply of the power supply voltage to the various circuits included in the semiconductor device 5400 is started. Note that a counter may be provided in the power controller 5402 to be used to determine the timing of starting the supply of the power supply voltage regardless of input of an interrupt signal. Next, the various registers and the cache 5404 start data restoration. Then, execution of an instruction is resumed in the control unit 5407.

Such power gating can be performed in the whole processor or one or a plurality of logic circuits included in the processor. Furthermore, power supply can be stopped even for a short time. Consequently, power consumption can be reduced at a fine spatial or temporal granularity.

In performing power gating, data held by the CPU core 5401 or the peripheral circuit 5422 is preferably saved in a short time. In that case, the power can be turned on or off in a short time, and an effect of saving power becomes significant.

In order that the data held by the CPU core 5401 or the peripheral circuit 5422 be saved in a short time, the data is preferably saved in a flip-flop circuit itself (referred to as a flip-flop circuit capable of backup operation). Furthermore, the data is preferably saved in an SRAM cell itself (referred to as an SRAM cell capable of backup operation). The flip-flop circuit and SRAM cell that are capable of backup operation preferably include transistors including an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region. Consequently, the transistor has a low off-state current; thus, the flip-flop circuit and SRAM cell that are capable of backup operation can retain data for a long time without power supply. When the transistor has a high switching speed, the flip-flop circuit and SRAM cell that are capable of backup operation can save and restore data in a short time in some cases.

An example of the flip-flop circuit capable of backup operation is described with reference to FIG. 33.

A semiconductor device 5500 shown in FIG. 33 is an example of the flip-flop circuit capable of backup operation. The semiconductor device 5500 includes a first memory circuit 5501, a second memory circuit 5502, a third memory circuit 5503, and a read circuit 5504. As a power supply voltage, a potential difference between a potential V1 and a potential V2 is supplied to the semiconductor device 5500. One of the potential V1 and the potential V2 is at a high level, and the other is at a low level. An example of the structure of the semiconductor device 5500 when the potential V1 is at a low level and the potential V2 is at a high level is described below.

The first memory circuit 5501 has a function of retaining data when a signal D including the data is input in a period during which the power supply voltage is supplied to the semiconductor device 5500. Furthermore, the first memory circuit 5501 outputs a signal Q including the retained data in the period during which the power supply voltage is supplied to the semiconductor device 5500. On the other hand, the first memory circuit 5501 cannot retain data in a period during which the power supply voltage is not supplied to the semiconductor device 5500. That is, the first memory circuit 5501 can be referred to as a volatile memory circuit.

The second memory circuit 5502 has a function of reading the data held in the first memory circuit 5501 to store (or save) it. The third memory circuit 5503 has a function of reading the data held in the second memory circuit 5502 to store (or save) it. The read circuit 5504 has a function of reading the data held in the second memory circuit 5502 or the third memory circuit 5503 to store (or restore) it in the first memory circuit 5501.

In particular, the third memory circuit 5503 has a function of reading the data held in the second memory circuit 5502 to store (or save) it even in the period during which the power supply voltage is not supplied to the semiconductor device 5500.

As shown in FIG. 33, the second memory circuit 5502 includes a transistor 5512 and a capacitor 5519. The third memory circuit 5503 includes a transistor 5513, a transistor 5515, and a capacitor 5520. The read circuit 5504 includes a transistor 5510, a transistor 5518, a transistor 5509, and a transistor 5517.

The transistor 5512 has a function of charging and discharging the capacitor 5519 in accordance with data held in the first memory circuit 5501. The transistor 5512 is desirably capable of charging and discharging the capacitor 5519 at a high speed in accordance with data held in the first memory circuit 5501. Specifically, the transistor 5512 desirably contains crystalline silicon (preferably polycrystalline silicon, further preferably single crystal silicon) in a channel formation region.

The conduction state or the non-conduction state of the transistor 5513 is determined in accordance with the charge held in the capacitor 5519. The transistor 5515 has a function of charging and discharging the capacitor 5520 in accordance with the potential of a wiring 5544 when the transistor 5513 is in a conduction state. It is desirable that the off-state current of the transistor 5515 be extremely low. Specifically, the transistor 5515 desirably contains an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region.

Specific connection relations between the elements are described. One of a source and a drain of the transistor 5512 is connected to the first memory circuit 5501. The other of the source and the drain of the transistor 5512 is connected to one electrode of the capacitor 5519, a gate of the transistor 5513, and a gate of the transistor 5518. The other electrode of the capacitor 5519 is connected to a wiring 5542. One of a source and a drain of the transistor 5513 is connected to the wiring 5544. The other of the source and the drain of the transistor 5513 is connected to one of a source and a drain of the transistor 5515. The other of the source and the drain of the transistor 5515 is connected to one electrode of the capacitor 5520 and a gate of the transistor 5510. The other electrode of the capacitor 5520 is connected to a wiring 5543. One of a source and a drain of the transistor 5510 is connected to a wiring 5541. The other of the source and the drain of the transistor 5510 is connected to one of a source and a drain of the transistor 5518. The other of the source and the drain of the transistor 5518 is connected to one of a source and a drain of the transistor 5509. The other of the source and the drain of the transistor 5509 is connected to one of a source and a drain of the transistor 5517 and the first memory circuit 5501. The other of the source and the drain of the transistor 5517 is connected to a wiring 5540. Although a gate of the transistor 5509 is connected to a gate of the transistor 5517 in FIG. 33, it is not necessarily connected to the gate of the transistor 5517.

The transistor described in the above embodiment as an example can be applied to the transistor 5515. Because of the low off-state current of the transistor 5515, the semiconductor device 5500 can retain data for a long time without power supply. The favorable switching characteristics of the transistor 5515 allow the semiconductor device 5500 to perform high-speed backup and recovery.

The structure described in this embodiment can be used in appropriate combination with any of the structures and methods described in the other embodiments.

Embodiment 6

In this embodiment, one mode of a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 34A and 34B and FIGS. 35A and 35B.

<Semiconductor Wafer and Chip>

FIG. 34A is a top view of a substrate 711 before dicing treatment. As the substrate 711, a semiconductor substrate (also referred to as a “semiconductor wafer”) can be used, for example. A plurality of circuit regions 712 are provided over the substrate 711. A semiconductor device of one embodiment of the present invention or the like can be provided in the circuit region 712.

Each of the circuit regions 712 is surrounded by a separation region 713. Separation lines (also referred to as “dicing lines”) 714 are set at a position overlapping with the separation regions 713. The substrate 711 can be cut along the separation lines 714 into chips 715 including the circuit regions 712. FIG. 34B is an enlarged view of the chip 715.

A conductive layer, a semiconductor layer, or the like may be provided in the separation regions 713. Providing a conductive layer, a semiconductor layer, or the like in the separation regions 713 relieves ESD that might be caused in a dicing step, preventing a decrease in the yield of the dicing step. A dicing step is generally performed while pure water whose specific resistance is decreased by dissolution of a carbonic acid gas or the like is supplied to a cut portion, in order to cool down the substrate, remove swarf, and prevent electrification, for example. Providing a conductive layer, a semiconductor layer, or the like in the separation regions 713 allows a reduction in the usage of the pure water. Thus, the cost of manufacturing semiconductor devices can be reduced. In addition, semiconductor devices can be manufactured with improved productivity.

<Electronic Component>

An example of an electronic component using the chip 715 is described with reference to FIGS. 35A and 35B. Note that an electronic component is also referred to as a semiconductor package or an IC package. For electronic components, there are various standards, names, and the like in accordance with the direction in which terminals are extracted, the shapes of terminals, and the like.

The electronic component is completed when the semiconductor device described in any of the above embodiments is combined with components other than the semiconductor device in an assembly process (post-process).

The post-process is described with reference to a flow chart in FIG. 35A. After the semiconductor device of one embodiment of the present invention and the like are formed over the substrate 711 in a pre-process, a back surface grinding step in which the back surface (the surface where a semiconductor device and the like are not formed) of the substrate 711 is ground is performed (Step S721). When the substrate 711 is thinned by grinding, the size of the electronic component can be reduced.

Next, the substrate 711 is divided into a plurality of chips 715 in a dicing step (Step S722). Then, the divided chips 715 are individually bonded to a lead frame in a die bonding step (Step S723). To bond the chip 715 and a lead frame in the die bonding step, a method such as resin bonding or tape-automated bonding is selected as appropriate depending on products. Note that the chip 715 may be bonded to an interposer substrate instead of the lead frame.

Next, a wire bonding step for electrically connecting a lead of the lead frame and an electrode on the chip 715 through a metal wire is performed (Step S724). As the metal wire, a silver wire, a gold wire, or the like can be used. Ball bonding or wedge bonding can be used as the wire bonding.

The wire-bonded chip 715 is subjected to a molding step of sealing the chip with an epoxy resin or the like (Step S725). Through the molding step, the inside of the electronic component is filled with a resin, so that a wire for connecting the chip 715 to the lead can be protected from external mechanical force, and deterioration of characteristics (decrease in reliability) due to moisture or dust can be reduced.

Subsequently, the lead of the lead frame is plated in a lead plating step (Step S726). Through the plating process, corrosion of the lead can be prevented, and soldering for mounting the electronic component on a printed circuit board in a later step can be performed with higher reliability. Then, the lead is cut and processed in a formation step (Step S727).

Next, a printing (marking) step is performed on a surface of the package (Step S728). After a testing step (Step S729) for checking whether an external shape is good and whether there is malfunction, for example, the electronic component is completed.

FIG. 35B is a perspective schematic diagram of a completed electronic component. FIG. 35B shows a perspective schematic diagram of a quad flat package (QFP) as an example of an electronic component. An electronic component 750 in FIG. 35B includes a lead 755 and the chip 715. The electronic component 750 may include multiple chips 715.

The electronic component 750 in FIG. 35B is mounted on a printed circuit board 752, for example. A plurality of electronic components 750 are combined and electrically connected to each other over the printed circuit board 752; thus, a circuit board on which the electronic components are mounted (a circuit board 754) is completed. The completed circuit board 754 is provided in an electronic device or the like.

Embodiment 7 <Electronic Device>

A semiconductor device of one embodiment of the present invention can be used for a variety of electronic devices. FIGS. 36A to 36F each illustrate a specific example of an electronic device including the semiconductor device of one embodiment of the present invention.

FIG. 36A is an external view illustrating an example of a car. A car 2980 includes a car body 2981, wheels 2982, a dashboard 2983, lights 2984, and the like. The car 2980 also includes an antenna, a battery, and the like.

An information terminal 2910 illustrated in FIG. 36B includes a housing 2911, a display portion 2912, a microphone 2917, a speaker portion 2914, a camera 2913, an external connection portion 2916, an operation switch 2915, and the like. A display panel and a touch screen that use a flexible substrate are provided in the display portion 2912. The information terminal 2910 also includes an antenna, a battery, and the like inside the housing 2911. The information terminal 2910 can be used as, for example, a smartphone, a mobile phone, a tablet information terminal, a tablet personal computer, or an e-book reader.

A notebook personal computer 2920 illustrated in FIG. 36C includes a housing 2921, a display portion 2922, a keyboard 2923, a pointing device 2924, and the like. The notebook personal computer 2920 also includes an antenna, a battery, and the like inside the housing 2921.

A video camera 2940 illustrated in FIG. 36D includes a housing 2941, a housing 2942, a display portion 2943, operation switches 2944, a lens 2945, a joint 2946, and the like. The operation switches 2944 and the lens 2945 are provided on the housing 2941, and the display portion 2943 is provided on the housing 2942. The video camera 2940 also includes an antenna, a battery, and the like inside the housing 2941. The housing 2941 and the housing 2942 are connected to each other with the joint 2946, and the angle between the housing 2941 and the housing 2942 can be changed with the joint 2946. By changing the angle between the housings 2941 and 2942, the orientation of an image displayed on the display portion 2943 may be changed or display and non-display of an image may be switched.

FIG. 36E illustrates an example of a bangle-type information terminal. An information terminal 2950 includes a housing 2951, a display portion 2952, and the like. The information terminal 2950 also includes an antenna, a battery, and the like inside the housing 2951. The display portion 2952 is supported by the housing 2951 having a curved surface. A display panel with a flexible substrate is provided in the display portion 2952, so that the information terminal 2950 can be a user-friendly information terminal that is flexible and lightweight.

FIG. 36F illustrates an example of a watch-type information terminal. An information terminal 2960 includes a housing 2961, a display portion 2962, a band 2963, a buckle 2964, an operation switch 2965, an input/output terminal 2966, and the like. The information terminal 2960 also includes an antenna, a battery, and the like inside the housing 2961. The information terminal 2960 is capable of executing a variety of applications such as mobile phone calls, e-mailing, text viewing and editing, music reproduction, Internet communication, and computer games.

The display surface of the display portion 2962 is bent, and images can be displayed on the bent display surface. Furthermore, the display portion 2962 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, an application can be started by touching an icon 2967 displayed on the display portion 2962. With the operation switch 2965, a variety of functions such as time setting, ON/OFF of the power, ON/OFF of wireless communication, setting and cancellation of a silent mode, and setting and cancellation of a power saving mode can be performed. The functions of the operation switch 2965 can be set by setting the operating system incorporated in the information terminal 2960, for example.

The information terminal 2960 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the information terminal 2960 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. Moreover, the information terminal 2960 includes the input/output terminal 2966, and data can be directly transmitted to and received from another information terminal via a connector. Power charging through the input/output terminal 2966 is also possible. The charging operation may be performed by wireless power feeding without using the input/output terminal 2966.

A memory device including the semiconductor device of one embodiment of the present invention, for example, can hold control data, a control program, or the like of the above electronic device for a long time. With the use of the semiconductor device of one embodiment of the present invention, a highly reliable electronic device can be provided.

This embodiment can be implemented in an appropriate combination with any of the structures described in the other embodiments.

This application is based on Japanese Patent Application Serial No. 2016-239044 filed with Japan Patent Office on Dec. 9, 2016, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a first insulator over a substrate; an oxide semiconductor over the first insulator; a second insulator over the oxide semiconductor; a first conductor over the second insulator; a third insulator over the first conductor; a fourth insulator in contact with a side surface of the second insulator, a side surface of the first conductor, and a side surface of the third insulator; a fifth insulator in contact with a top surface of the oxide semiconductor and a side surface of the fourth insulator; and a second conductor in contact with the top surface of the oxide semiconductor and the fifth insulator, wherein a level of a top surface of the fourth insulator is higher than a level of a top surface of the fifth insulator.
 2. The semiconductor device according to claim 1, wherein an angle formed between the side surface of the first conductor and a bottom surface of the oxide semiconductor is greater than or equal to 80° and less than or equal to 100°.
 3. The semiconductor device according to claim 1, wherein upper corners of the oxide semiconductor are rounded in a cross section, and a curvature radius of each of the curved corners is greater than or equal to one-fourth of a thickness of the oxide semiconductor and less than or equal to the thickness of the oxide semiconductor.
 4. The semiconductor device according to claim 3, wherein the second conductor is in contact with the corner, the top surface of the oxide semiconductor and the fifth insulator.
 5. The semiconductor device according to claim 1, wherein first electrostatic capacitance is formed between the oxide semiconductor and the first conductor, wherein second electrostatic capacitance is formed between the first conductor and the second conductor, and wherein the second electrostatic capacitance is smaller than the first electrostatic capacitance.
 6. The semiconductor device according to claim 1, wherein the oxide semiconductor comprises In, an element M, and Zn, and wherein the element M represents Al, Ga, Y, or Sn.
 7. The semiconductor device according to claim 1, wherein the oxide semiconductor comprises a first region and a second region, wherein the second insulator overlaps with the second region, wherein the first region is in contact with the fifth insulator and the second conductor, and wherein a concentration of at least one of hydrogen and nitrogen is higher in the first region than in the second region.
 8. The semiconductor device according to claim 7, wherein the fourth insulator and the second region overlap with each other.
 9. The semiconductor device according to claim 1, wherein the fourth insulator comprises aluminum oxide or hafnium oxide.
 10. The semiconductor device according to claim 1, wherein the third insulator comprises aluminum oxide or hafnium oxide.
 11. The semiconductor device according to claim 1, wherein a thickness of the third insulator is greater than a thickness of the fourth insulator in a direction parallel to a top surface of the substrate.
 12. The semiconductor device according to claim 1, wherein the first conductor comprises a conductive oxide.
 13. The semiconductor device according to claim 1, wherein the fifth insulator comprises at least one of hydrogen and nitrogen.
 14. A method for manufacturing a semiconductor device comprising: forming a first insulator over a substrate; forming an oxide semiconductor over the first insulator; forming a first insulating film, a first conductive film, and a second insulating film in this order over the oxide semiconductor; etching the first insulating film, the first conductive film, and the second insulating film to form a second insulator, a first conductor, and a third insulator; forming a third insulating film to cover the first insulator, the oxide semiconductor, the second insulator, the first conductor, and the third insulator; performing first dry etching treatment on the third insulating film to form a fourth insulator in contact with a side surface of the second insulator, a side surface of the first conductor, and a side surface of the third insulator; forming a fourth insulating film to cover the first insulator, the oxide semiconductor, the fourth insulator, and the third insulator; performing second dry etching treatment on the fourth insulating film to form a fifth insulator in contact with a side surface of the fourth insulator; forming a sixth insulator over the first insulator, the oxide semiconductor, the third insulator, and the fifth insulator; forming a first opening and a second opening in the sixth insulator; forming a second conductor to fill the first opening; and forming a third conductor to fill the second opening, wherein part of the fifth insulator and at least part of a top surface and a side surface of the oxide semiconductor are in contact with the second conductor in the first opening.
 15. The method for manufacturing the semiconductor device according to claim 14, wherein the third insulating film is formed by an ALD method, and wherein the fourth insulating film is formed by a PECVD method.
 16. The method for manufacturing the semiconductor device according to claim 14, wherein the first conductor is a gate electrode, wherein the second conductor is a source electrode, wherein the third conductor is a drain electrode, and wherein the source electrode and the drain electrode do not overlap with each other when seen from a direction parallel to a channel length direction.
 17. A semiconductor device comprising: a first insulator over a substrate; a first oxide semiconductor over the first insulator; a second oxide semiconductor over the first oxide semiconductor; a second insulator over the second oxide semiconductor; a first conductor over the second insulator; a third insulator over the first conductor; a fourth insulator in contact with a side surface of the second insulator, a side surface of the first conductor, and a side surface of the third insulator; a fifth insulator in contact with a top surface of the second oxide semiconductor and a side surface of the fourth insulator; and a second conductor in contact with the top surface of the second oxide semiconductor and the fifth insulator, wherein a level of a top surface of the fourth insulator is higher than a level of a top surface of the fifth insulator, wherein each of the first oxide semiconductor and the second oxide semiconductor comprises In, an element M, and Zn, wherein the element M represents Al, Ga, Y, or Sn, and wherein an atomic ratio of In to the element M in the second oxide semiconductor is greater than an atomic ratio of In to the element M in the first oxide semiconductor.
 18. The semiconductor device according to claim 17, wherein the second oxide semiconductor comprises a first region and a second region, wherein the second insulator overlaps with the second region, wherein the first region is in contact with the fifth insulator and the second conductor, and wherein a concentration of at least one of hydrogen and nitrogen is higher in the first region than in the second region. 